What kind of bike do you want:
Stock 26" wheels( the bigger bikes,or
A mod 20" wheels the more bmx ones.
Both are good but mod are more for comps or you can get smaller bikes that are street. If you want a mod get a t-pro. or try and get a second hand t-lite if you want a more streey bike as they are good plus small. the brand new price of a t-lite is £600 and t-pro is £420 , which is the best bike to buy really to start on.
O rstock I don't no lots about but you should get a onza t-rex for £500. all these bikes come from supercycles.co.uk or if you want a 24" bike a onza zoot comes out soon and it is a smaller version of the t-rex most people say.
hope this helps.
toby