I've been temporarily looking for a job in the UK so I can point you to some of the sites I checked or that were advised here:

http://hackerjobs.co.uk/ It was recently launched, you'll find some posts they made here on HN.

http://www.theitjobboard.co.uk/ I've found some interesting job openings here.

You should also check the monthly post "Who's Hiring" here on HN they have some openings in London. I think I've seen at least one each month.

http://news.ycombinator.com/item?id=4323597 August version

EDIT : I've kept a list of companies that I was interested in and had openings in may/june, would you be interested to have it?

EDIT2: Added link to august discussion "Who's Hiring"