Don’t Streetcars and Zipcars already operate on similar lines? I’d consider them but the sign up fees are too high. Best short term car hire deal I got was from SixT – three hours’ hire for about £11. That gave me enough time to drive from Victoria to Crystal Palace, pick up a desk I purchased on ebay, drop it off at home and return the car. Much cheaper than a taxi but I don’t think they offer it any more.
Couldn’t have done it with one of those French Autolibs – they’re far too small.