Jessops in Oxford Street always have a good selection (as do most Jessops shops).

If you fancy a drive, how about a trip to Sussex (Haywards Heath) to see Park Camera. Got my 1V, 70-200L (non-IS) and my 100-400L IS from them a couple of years ago. Always have a good stock and reallyknowledgable folks.

I saw an EOS 1Ds on email for 2000quid last week. Now that's a bargain as it retails for 7k. Not bad for a 11mp camera :-)