In reply to JMGLondon:
When I lived in the UK I used to use fetcheveryone.com as they have a race finder facility.
Since you're in London, have you looked at the Box hill Fell Race? I did it once it was great fun - around this time of year.
Trionium do some good stuff too - Midsummer Munro, Greensand Marathon and the Pilgrim Ultra - they're not cheap but OK if you're only doing a few and not racing every week they're worth a try.