I raced last year on a £20 gumtree GT, as long as your gearing is ok anything will do to start. No restrictions on tracks, just no pegs, rear brake, long sleeves and legs covered + a full face helmet and gloves.
A race specific frame will have a longer rear end, Haro do a decent entry bike.
Here's mine for this year - another gumtree deal and my first ever bike with carbon forks! Guy had put the wrong crown race on when building it up so I reckon he thought it was broke.