According to this Bloomberg article the number of Americans in the UK has decreased by 4% over the past year.
Britain is raising tax rates (up to 50% now for those making more than 150KGBP/annum), the pound is weakening, the job market is shrinking, and maintaining a certain standard of living here requires far more money than back in the USA.
The economic picture is also gloomier in Britain. The U.K. economy shrank 5.6 percent in the year through June, compared with 3.9 percent in the U.S. London’s financial industry lost 29,371 jobs, or 8.3 percent of the total, last year, according to the Centre for Economic and Business Research. - Bloomberg
People who are tired of paying high taxes and having a lower quality of life are ditching the UK to head back to the States - and expats from other countries are leaving as well. Things here are not looking so hot - politicians elsewhere looking to stifle the economy by raising taxes should take note.