sharmanian - there's the Isle of Wight fell running festival in September.
Also, a lot of the races in the south Peak district are a 3 hour drive from London on a good day - depends what part of London your're in though, but something like the Black Rocks fell race (which is a weekday evening in July) would be do-able - leave work at lunchtime, drive to Cromford (Junction 27?28? of M1), do race (starts 7:00/7.30 p.m., go to pub, leave Derbyshire at 9/9:30 ish, back just after midnight - easy.
Or a chap called Dave Denton organises fell races in Milford and Ambergate, which are even further south
(just north of Derby in fact) www.runningwithdavid.com and definitely within day trip range