http://www.treehugger.com/bikes/shareroller-portable-turbo-kit-electrifies-any-bike-sharing-bicycle.html

Clip on, roller motor for the front tyre of a London BorisBike or similar. 
Interesting idea. But I'm not at all sure how feasible it is. The design is for a 750W version, but would have to be 250W for the UK/EU market.