Running is obviously accessible; cycling a bit less so, given the direction racing bike prices are heading (20 years ago, £1,000 would have bought you something truly outstanding with Dura-Ace kit; now you'd need many times more and transport to events is impossible without very understanding and well-off parents). Track cycling is only accessible if you happen to live near a track, which rules out most of the country although the London velodrome will make a huge difference,.
In the end, the blue riband event is the 100m sprint, and anyone can have a go at that. I think my official time at secondary school was 18 seconds. Usain Bolt has nothing to worry about.