Turbo fit are at Tanners Lane in Blakelands in Milton keynes (http://www.turbofit.co.uk) then TDI Motorsport is next door to Big Boys Toys in Thurrock next to Ikea, for 500BHP you would be looking at a small single turbo me thinks, I would go to Turbo fit rather than TDI for that as the price difference would be HUGE!!
I am 2 mins from TDI so if you need some advice give me a PM and we could meet up, I have a HKS To4R single built by TDI and have had a GTR R32 built by them also (R32 ran 735 BHP), good guys but expensive, I would only use turbo fit now
See pics for TDI's work i have had done, the R32 cost £19,700! for the engine, ECU, Fueling and exhaust alone