Well Audi partner with Stasis for some tuning I know in America, not sure about over here. You can get a Stasis tune and exhausts from the dealers. I have seen a Stasis R8 GT, so there must be somewhere here.
Otherwise, the brands preferred by the VW/Audi scene seem to be Revo/APR for tuning, Milltek for exhaust, K&N for filters, Carbonio for intakes, KW/FK/Eibach for suspension, OEM/VMR for wheels, Brembo for brakes, and OEM+ for everything else.
If I were you, and had the money for it, I'd got for a full DTM conversion for the body (OEM+), 19" VMR 710s for wheels, KW V1 coilovers for suspension, RS4 brakes (Brembo & OEM+), and a Milltek exhaust. But that lot would run to £4500-5000 if your lucky.