Farnell: Good selection of individual electronic components in the UK, but smaller suppliers may be cheaper for some popular items.

CPC: Sister company to Farnell. Good for tools, general electrical and hobby electronics. Again, some items might be cheaper elsewhere.

http://www.bitsbox.co.uk/: Basic (traditional) components for hobbyists. Low delivery charges and good service.

Hobarts: Laser-cutting materials.