Generally in London it is only worth getting off the tube if there are four chain charity shops, but I am always prepared to get off if there is what I call Silverfish Special, On average I have to visit ten charity shops to find one item to buy, but then I am renowned for being a miserable mean bugger.
If you walk in an area you are only going to get to visit about six charity shops. If you go by bike then it is possible to build up a circular route near you. If you go by bus, you are going to end up in parts of London you never knew existed, if you go by car you are never going to be able to park, the only sane speedy way of getting around London is by tube and train. You should just ignore the distances and think where is the biggest and best group, and at the moment this is definitely the area around Watford and Chesham. It is middle class areas like these that can get the volunteers, not the posh areas like Mayfair or the working class ones like Brixton, the best areas tend to be on the periphery of London.