I've used Lanway for the past three years. They're very good, they can supply just about anything, build PCs to a spec, are very quick to deliver and very well priced.
They're based in Lancashire, though I don't know what geographical area they cover.
They also install cabling and set up netowrks for those starting from scratch.
(No, I don't work for them. They're just a good supplier worth a mention).