I missed this at the time, but this IHT article, For non-Britons living in U.K., price of residency gets heavier, suggests the massive increase in immigration fees forces legal immigrants to pay for efforts to police illegal immigrants.
I know I’m an unusual case because I work for a charity and earn a lot less than a normal IT person, but Britain has benefited economically a lot more from my being here than I have. I had to save throughout the year to pay the fee and I’m still not sure it’s worth it.

Officials say the proceeds will help pay for a big new push to enforce immigration laws and crack down on illegal arrivals. The Home Office, the government department in charge of domestic security, said it wanted to hire more enforcement agents, build detention centers and increase its ability to process migrants efficiently without spending tax money.
Immigration Minister Liam Byrne said it was fair to require those who benefit economically from living in Britain to pay for the changes. The Home Office emphasized that the biggest increases were directed at foreigners who live and work in the country, not at students or visitors, and said it was fair to raise charges now because they had not gone up since April 2005.
But immigrants say they are being unfairly burdened with the cost of an effort that was previously shared by all British taxpayers, not just the foreign-born ones.