www.ebuyer.com is the only one I'd go so far as to recommend. Huge stock, good prices and reliable. User comments worth reading too, there's lots of the usual stupid comments but say one out of every ten make it well worthwhile.
The downside to ebuyer is they aren't so good with their range of premium parts, though actually they've got better as the site is a bit more consumer orientated (previously was more a trade place really). I've never had to return anything but a friend has, he made a point of saying how good they were (though the part was clearly faulty and he was expecting a fight, YMMV).
Only other one I'd consider using that hasnt already been mentioned is:
http://www.overclockers.co.uk/
who seem to specialise in premium parts. Bought a couple of orders from them, no problems.
oh and props to
http://www.dorothybradbury.co.uk/ for fans. Website looks awful but I bought a handful of Panaflo's from her a few years ago, one was noisy so she replaced it straight away without me having to send back the duff one.