I live about 1nm north of the City tower and notice occasional SE traffic, maybe one or two a weekend, generally due N-S and overflying the tower. However there isn't the frequency there was some years ago.
Lea Valley may be an option north of the Thames but there is no equivalent to continue along south of the river. And if you are going to turn along the Thames you might as well have followed the M25.
From Stapleford I do occasional trips down towards Surrey but always have gone via Dartford Bridge and past Biggin. Anyone feel I'm over-cautious? I don't.
As an aside the BBMF Spits come over from time to time in the summer. Are they exempt from the "Land Clear" rule?
And as another aside about landing clear, when flying over entirely wooded country in Florida my instructor said "Engine failure, put it down on a straight road with no telephone poles. If there isn't one, go for a lake!".