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.
http://tech.slashdot.org/story/14/03/02/2222223/invention-makes-citibikes-electric
http://gizmodo.com/how-it-feels-to-ride-an-electric-citibike-1533651538
https://www.google.com/search?q=shareroller&oq=shareroller&aqs=chrome..69i57j69i59j69i61j69i60.3955j0j4&sourceid=chrome&espv=210&es_sm=93&ie=UTF-8#q=shareroller&tbm=nws
Unfortunately, so are the uninformed commentators.