City of London
A list of toilets in the Square Mile.
http://www.cityoflondon.gov.uk/Corporation/our_services/health_safety/public_toilets.htm

Last Rounds.co.uk
An interactive toilet map of London.
http://www.lastrounds.co.uk/public_toilets.html

Need A Loo?
Lists publicly accessible disabled toilet facilities throughout the UK.
http://www.needaloo.org/

Smooth Hound Systems
Advice on finding public toilets in the UK.
http://www.s-h-systems.co.uk/tourism/advice3.html#toilets