I've used Chain Reaction, Wiggle, Parker, Prendas, Butlers, ProBikeKit
Also a couple of German places - eg [url=http://www.starbike.com/]StarBike[/url]for all that lightweight stuff like Tune
Ebay shops - [url=http://stores.ebay.co.uk/DHCycleSport_W0QQssPageNameZstrkQ3amefsQ3amesstQQtZkm]DHCycleSport[/url] - who are NOT Dave Hinde - in the UK and [url=http://stores.ebay.co.uk/jedi-Sports_W0QQssPageNameZstrkQ3amefsQ3amesstQQtZkm]JediSports[/url] in Germany who I just bought my new wheels from - very good on wheels and groupsets
There is a (sometimes) useful UK weborder comparison website at
http://www.bikepimp.co.uk