AOG
Part of the reason will be because of our burgeoning IT industry, and the City. London in particular is a huge international financial centre, with hundreds of investment banks, insurance companies, futures traders etc...all of whom have an international presence and whom have staff coming into (and going out of) the country on short term contracts.
In addition to this, we are a key destination for Eastern Europeans who want to work in the services, domestic, building trades.
Over the past eight years, we've had our house renovated/redecorated twice, and had electricians, plumbers, carpenters etc round - and on most occasions, the accent we hear is Eastern European.
I assume (without much knowledge of the economies of other European countries) that we offer decent salaries for skilled workers.
Perhaps our economy is that much more stable?