Guardian's jobs site would be a good start http://jobs.guardian.co.uk/index.html
You can also register yourself with a few specialised recruitment agencies. Checking the websites of relevant companies' websites also would be a good idea. Most advertise jobs on their vacancy pages, and some state that they accept applications from recent graduates (some big companies have graduate schemes, that would give a start-up grant for you to settle, find a place to rent etc.)