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"
Your degree won't qualify you as a highly skilled worker, so unless you have other connections you're probably in for a long hard slog getting a suitable visa.
An Advanced UK-issued degree, relevant to the appointment, may be an exception. But absent that, the general rule on "expertise" ~= you are the only person in the EU with such. Alternatively, if your appointment is at "Board Level" (Ceo, Director, etc.) there is an exemption, so you may qualify as a "co-founder" (if such an opporunity were to arise).
The devil is in details, however. Granular and somewhat varying year-to-year.