After building a kiosk and adding burgers and shakes to the menu in 2001, success with diners drove the Danny Meyer-led company to expand. In 2013, Union Square Hospitality Group opened the first UK Shake Shack location in London's Covent Garden area.Rachel, Insider's New York-based food reporter, headed to her local Shack inside of a transit center in lower Manhattan.The ordering station and open-kitchen concept are married into one at this location, as they are at many stateside Shake Shacks. There's an inside seating area for dine-in customers, but she chose to take her meal to go. Meanwhile, Insider's senior lifestyle reporter Mikhaila headed to her local Shake Shack at London's Covent Garden. The ordering station is located inside, with a seating area outside. Since the weather was nice, she opted to sit outside and order her food through the website, which you can do by scanning the table's barcode on your phone.The ShackBurger we had in New York cost $6.19, or around £4.55, while in the UK, it cost £5.95, or around $8.12.This burger had minimal ingredients, with both sandwiches featuring a toasted bun, a 4-by-4-inch beef patty topped with melted cheese, two tomato slices, and lettuce.In the UK, the burger, bun, tomatoes, and lettuce were all generously covered in the brand's famous ShackSauce, Mikhaila said. But Rachel found just a small spoonful on the top bun in New York. The UK seemed to be more generous with sauce and with lettuce as the green leaf was bursting out of Mikhaila's sandwich. This burger had slightly more ingredients than the ShackBurger. The burger we tried in the US cost $7.69, or around £5.66, while the UK version cost £7.75, around $10.57.The burgers were both topped with melted cheese, two strips of bacon, and sprinkled with red cherry peppers. Mikhaila said her sandwich had too many peppers, as they started to fall out of the burger with each bite. Rachel didn't have that problem.The SmokeShack also featured ShackSauce, which was equally scarce on both sides of the Atlantic.A bold, smoky flavor came from the bacon, which made up for the lack of sauce and added a profile that differentiated this sandwich from the others on the menu. The 'Shroom Burger consisted of a fried portobello mushroom with melted cheese, lettuce, two tomato slices, and ShackSauce.The UK version cost £5.95, or around $8.12, and weighed 250 grams (8.82 ounces). At roughly 2.5 inches wide and 3 inches long, the 'Shroom Burger was slightly smaller than the previous patties both reporters ate. Rachel's cost $7.89, or around £5.80 in New York, and was topped with the same ingredients, though the tomatoes used on her sandwich were visibly smaller than on Mikhaila's.The Shack Stack is described on the menu as a cheeseburger and a 'Shroom Burger topped with lettuce, tomato, and ShackSauce.The UK version cost £8.95, or around $12.21, and weighed 284 grams (10 ounces). It contained a beef burger slathered in melted cheese, a fried portobello mushroom stuffed with more cheese, two slices of tomato, and lettuce. In the US, it cost $10.99, or around £8.03, and weighed 9.75 ounces (275 grams). It contained the same elements as the one in the UK, but the tomatoes and lettuce were smaller in New York. Similar to the Smoke Shack, the ShackSauce was placed sparingly and was only visible on the top bun and lettuce. Mikhaila said more sauce would have made all the difference here, as this burger didn't seem to have as much flavor as the other burgers she tried. Rachel agreed, and added that the beef patty didn't add much of anything except more weight. The UK version was £6.95, or around $9.50, and had three pickles, shredded lettuce, buttermilk herb mayo, and fried chicken. The piece of chicken was 3 inches wide and 4 inches long.Mikhaila has never been a fan of pickles, but that didn't stop her from appreciating the taste of the chicken, which appeared to be seasoned with pepper.Similarly, the seasoned piece of fried chicken that Rachel was served in New York was flavorful and featured the same toppings, though not in the same amounts. Rachel's sandwich had two large slices of pickles and just a few strands of shredded lettuce. While she said the lettuce was disappointing, the crispy, 4.75-by-3.5-inch chicken breast was impressive. It cost $7.49, or around £5.47.A six-piece order of the restaurant's nuggets costs $4.99, or around £3.67 in NYC, and comes with a side of either honey mustard or barbecue sauce. The six chicken pieces weighed 3.25 ounces (92.5 grams) together — though some were much larger than others. Rachel noticed that the breading on her New York nuggets was falling off in places, though it was still impressively crispy. Mikhaila didn't have the same issue, and her nuggets had an equally soft and crispy texture.The chicken nuggets she ordered in London cost £4.75, or around $6.50, and weighed 99 grams (3.49 ounces). While the menu described the order as containing six chicken bites, Mikhaila was given seven, and they looked to be more uniform in size than their stateside counterpart.A signature move for Shake Shack is to cut their flat-top-cooked hot dogs lengthwise. This allows for a larger grilling surface area and adds some additional flavor. The chain serves it plain in a bun with no additional toppings or sauces. In the UK, the dish cost £4.50, or around $6.14, and was 6.5 inches long. In the US, it cost $4.09, or around £3.01, and measured out to be 6.75 inches long. Rachel's order of fries weighed in at 6.5 ounces (184 grams) and cost $3.15, or around £2.32. Mikhaila's fries weighed considerably less at 94 grams (3.3 ounces) and cost £3, or around $4.08 — which means, if this is consistent across servings, customers in the UK are getting a worse deal on their fry orders.Looking closely at the fry shape and crinkle pattern, the US and UK fry offerings look slightly different, with the US version appearing thicker in the center.Without cheese sauce to keep them moist, both the US and UK fries seemed to get cold and hardened quickly after purchasing. In the US, Rachel ordered her cheese fries to-go, so the downtown restaurant packaged the sauce separately from the fries. She thought it made sense considering the cheese would have gotten on the other items, but then realized it meant she got less than she may have if it was poured directly onto the potatoes from the onset. With the cheese on top, her serving — which cost $4.75, or around £3.49 — weighed 8.5 ounces (240 grams). Meanwhile, Mikhaila ate her fries at the restaurant. They cost £4, or around $5.44. Similar to the regular fries, the UK cheese fries weighed less than the US version, at 178 grams (6.2 ounces). When Rachel tried her cookies and cream milkshake, she first noticed that there was still 3/4-inch of room at the top. That's at least two or three sips of milkshake by her own measurements. In total, the sweet treat, which cost $5.59, or around £4.11, weighed 13.5 ounces (385 grams). The flavor Rachel tried was not on the UK menu at the time of writing. She said it was reminiscent of Oreo cookies, having detectable bits of black-colored cookie and soft bites that reminded her of that familiar Oreo creme filling. It was creamy and frothy in texture on top and around the edges of the cup, and had a more viscous and frozen center. Mikhaila tried the £4.95, or around $6.73, caramel milkshake, which wasn't available in the US at the time of writing. She noticed a similar problem to Rachel — there was room for more servings in the cup.Our UK reporter is a longtime fan of Shake Shack's milkshakes due to the creamy and smooth texture, and the caramel offering was no different. The items you can get exclusively in the UK include the caramel milkshake, the ShackMeister, and the vegan crispy shallot burger. As well as the caramel shake — which ended up being our UK reporter's favorite item she tried — Mikhaila also ordered the ShackMeister, which didn't live up to expectations. She was unable to try the vegan burger at the time of reporting.Crispy shallots are the main ingredient in this item, which became evident to Mikhaila upon her first couple of mouth-fulls — she couldn't taste anything else.The ShackSauce was drizzled lightly on the top bun and, to her disappointment, only on one side of the burger. It cost Mikhaila £6.75, or around $9.18.The menu items currently available in the US and not the UK are bacon cheese fries, grilled cheese, a plain hamburger, and the cookies-and-cream-flavored milkshake.Though she's been to Shake Shack many times before, Rachel hadn't actually tried any of these items before. Rachel's never been a huge fan of the Shake Shack crinkle-cut fries. She says they are typically cold and cardboard-like within minutes of purchasing. But, the cheese and bacon did add some nice texture and flavor.This order weighed in at 9.25 ounces (265 grams) and cost $5.15, or around £3.79.This patty was the same size as the others, and the bun was just as toasted. Rachel found the super-thin patty to be dried out and offering minimal flavors other than salt. The soft, always-tasty Martin's potato bun was the savior here. The sandwich weighed 4.75 ounces (135 grams) and cost $5.59, or around £4.11.With what looked like just one slice of cheese melted onto an inside-out bun, this grilled cheese sandwich is as simple as can be. But the flavors and textures of the griddled bun combined with the gooey cheese were definitely enough to satisfy someone looking for a meal reminiscent of childhood lunchrooms, and there's nothing wrong with that. It was the lightest sandwich option at 2.5 ounces (70 grams) and cost $4.49, or around £3.30.While a lot of the menu options were similar in size and toppings, there were discrepancies when it came to the amount of sauce used and the size of the toppings. These differences, though, can be boiled down to the produce each location had available at the time and the employee who was dishing out the sauce. Another glaring difference was the serving size of each fry order. The US orders consistently had more potatoes per serving, and they cost less than their UK counterpart, which also followed another theme of differences. Across the board, customers in New York City — which is an area known for high prices in the US — were paying less for their meals. In some cases, US customers are even getting more food for a lower price than those in the UK.