kelkoo
http://www.kelkoo.co.uk/

Quickly compare prices from a range of e-tailers for a whole range of goods, not just computer stuff.
It's not comprehensive though, and it can still be worth searching further afield.

dabs.com
http://www.dabs.com/

Vast range of computer equipment at very competitive prices. The site has recently been redesigned, but is still very easy to use. Usually very reliable, but if things go wrong their Customer Services can be a bit unhelpful, but for that range at those prices I always end up forgiving them

B & Q
http://www.diy.com/

Yes, the B & Q. Much the cheapest place to buy wired networking supplies. Things like Cat5 faceplates, which cost around £20 from computer suppliers, are 2-3 quid in B & Q

PC World
http://www.pcworld.co.uk/

Used to be expensive, but it's always worth checking their prices these days. Mind you some of the stories I hear about their Technical Support ...

7dayshop.com
www.7dayshop.com

Cheap printer cartridges, etc.

Strolling Guides Photography
www.strollingguides.co.uk

Unashamed plug for my other site.

Photographs and guidebook articles covering the UK, especially the New Forest, Cornwall, Dorset and Wiltshire.

Fuzioned
fuzioned.com

'Hosting for the Happy' and they mean it. Excellent support, first class reliabilty, and low prices ... what more could you ask for from a web host