Coming to London at this point of time is not a wise decision, not that I mean to discourage you. The job market is quite competitive because loads of developers are in the markets, because of the layoffs. So be ready to wait for some time to land a role.
Banks aren't recruiting that heavily, but that might change if the economy picks up. Regarding salaries, the contract rates you quote are primarily for banking sector jobs, some outside banking also pay those rates, but they are few.
You can quote what you want to a recruiter, most contracts are through them as most managers have a fincancial get go between recruiters and themselves. Recruiters take their cut what they bill, 400+200(just a guess). So the more they take from the 400, better is their margin. So they try to decrease the 400 portion.
But the important point is be ready to keep your chair warm for some time. I am not sure why you have to move to London. Keep your current job. Get a Skype number or something and get the calls diverted to your phone in Germany. You can come down to London for interviews and schedule them so you come in a week and give all your interviews. London is a costly place, you can find cheap places to stay too. But without a job and searching for one will get you depressed(been there and experienced it)