There's Thor racing, mainly toyota's but i think they're pretty good. MGT Racing are suposed to be good also but specialise more in skylines. Further afield you have optimium performance in Brackley northants and MG Autos towards derby way.
I've used Tolman motorsports in the past to, highly recommend these guy's he know's his stuff, they're in between coventry and rugby.
If you're having problems with bit's you can also give me a shout, i'm in coventry and know a reasonable amount on evo's, i do all my stuff myself, the only reason i had to go to tolman was to get a gearbox rebuild as it's not something i've done before and was in a rush!
Cheers Andy