Lots of green park areas around and leading up to the Vauxhall area.
Failing that.... a very, very,very slow descent onto one of the bridges ? (very much less than ideal, I know.... but better than meeting a crane in the mist !)
Oh, and there's also the Oval cricket ground !
yeah of course.. any heli can just land at will in a park in built up area like inner city london, have a smoke and wait for the weather to clear up.