I work for a large college in Coventry i'm a music tech.
although we haven't used these guys for any systems of our own, loads of students and staff here used a local company

www.inta-audio.com

They have had their pc's in Sound on Sound, they got really good reviews and are well priced, if you don't fancy the challange of building from scratch!

If you can afford a bit more, Philip Rees the guys who made all those wacky midi boxes are building kick ass pc's for audio which would scare the shit out of carillion!