I suppose you'd need someone with geographical knowledge of the London area to help determine a good location, however, I'd refrain from placing a stash in an area where you are not able to maintain it on a semi-regular basis. The stash owner needs to be able to visit his/her stash occasionally to verify it still exists at the designated coordinates, hasn't fallen victim to plunderers or the elements, hasn't become exposed due to unforseen circumstances, etc.
If you travel to London on an annual or semi-annual basis, or know someone there who can maintain it for you, go for it.
Good luck.
Matt
> I am traveling to London this week and want to place > on there, but need advice on where (in an urban > environment) to place one??? Can you give ideas?