A colossal number of people moved to the UK last year - and most of them are from just 4 countries

The UK just recorded a massive year for migration. People are streaming into the UK, mostly from Europe, at the fastest pace on record, according to figures released Thursday.

Most headlines will broadcast the net migration figure, which rose by 318,000 in 2014 - the biggest since statistics began in 1970. The net figure subtracts the number of migrants who left the country in 2014 from the total number of people who migrated to the UK in the same year. A total of 641,000 people immigrated to the UK in 2014.

That overwhelming figure actually understates the huge number of people who registered for National Insurance numbers (used in the UK to prove you're properly registered to work for tax purposes): A whopping 824,154 more people got NI cards in the year to March 2015, figures released Thursday morning show. That number is equivalent to about 1.2% of the UK's population, with 629,410 NI numbers issued to people from other EU countries. Of those, most are from just four countries:

• Poland: 115,606
• Romania: 152,363
• Italy: 57,635
• Spain: 54,203

This is a seriously massive year. There were about 120,000 more NI numbers issued over the last year than in the next-highest year since the financial crisis (in the year to March 2011 just over 700,000 numbers were issued).

It's not clear why the NI registration figure is so much considerably larger than the registered migration figure. The periods are slightly different (the net migration figures are for 2014 as a whole, while the NI number figures are for the year to March 2015) but that doesn't really account for another 180,000 or so people coming to the UK.

It's clear to see from a map of registrations that while there are urban hubs of migration in places like Manchester and Birmingham, the applications are located primarily in the south east of the country and London (in fact, those two areas combined make up more than half the total):

The concentration is so obvious that many London boroughs recorded more applications than huge areas of the UK: 26,480 registrations were made in the borough of Newham, against just over 22,000 in Wales and the North East of England put together.