Over the Thames
I think you will find that the precident of this situation has already been recorded in Court,and that the Thames comes into the same arena as :- Parks,Sports fields,and other areas open to the public situated in built up area's.
The situation is different in the take off / landing case,but an overflight of a built up area in a single is always going to be difficult to justify,and indeed the description 'of a suitable place' anywhere is open to many variations that you may not want to explore from the 'dock'.