www.meshcomputers.com are cheap so are
www.novatech.co.uk.
You can try out
www.overclockers.co.uk they have been known to have CS issues though and are mainly a PC parts place but they do, full systems.
Also
www.evesham.com another wicked maker many raving reviews from them.
Personally I have a Mesh computer from
www.meshcomputers.com and I love it never let me down over the week or so now I have had it runs everything and CS staff happy to help whenever they can.