As far as I can tell, the only landing areas would be William Girling and King George V reservoirs, the reservoir complex at Tottenham, Walthamstow marshes (where Mr A V Roe used to crash !!!) and Hackney marshes.
Hackney marsh is the only realistic area, but it does have the disadvantage of being totally covered in football pitches which are in regular use.
I live in NE London and quite often walk along the Lee Navigation, so I know the area very well from the ground (admittedly not from the air) and I honestly can't see how this route could be said to meet the land-clear rules.