Just a few links to sources of parts for your 3lb machine. They are based in the US, but are more than happy to ship the the UK.....

http://www.robotmarketplace.com/store.html
http://www.dimensionengineering.com/
http://banebots.com/index.html
http://www.kitbots.com/
http://www.pololu.com/
http://www.fingertechrobotics.com/

Based in the UK...

http://technobots.co.uk/
http://www.bearingboys.co.uk/
http://directplasticsonline.co.uk/
http://www.hpcgears.com/
http://www.metalfast.co.uk/
http://store.modelpower.co.uk/

And finally a very useful tool to help you calculate all aspects of your drive train and battery life.

http://architeuthis-dux.org/torquecalc.asp

(Posted for everyones benifit, I have no comercial interest in any of the above)