This thread has, as ever, drifted off subject a bit. I suspect that the Spitfires in the flyby were military and thus not subject to "congested area" rules a la CAA. Simply put, different rules for different rulebooks. Fair enough.
I am a bit more concerned about the discussions of singles flying over Greenwich and Blackheath, as foxmoth said it aint legal just cos Thames said OK.
The legal definition of "congested area" appears to include everything except open farmland and countryside. (can someone with an ANO provide the full definition please?) It certainly includes such "open land" as golf courses, parks, commons, playing fields (occupied or not) etc, and probably could be construed to include the River Thames itself. As Pinger said, if Hyde park is not open land then neither is any other patch of grass surrounded by houses...the "island of open land" does not exist within the metropolis.
A light aircraft flying VFR over Greenwich at it's max alt of, what, 2400'? would need a pretty stunning glide performance to "alight clear" somewhere close to the M25 in order to comply strictly with the above. Now it is clear that the CAA do not chose to apply this to the letter at present, but woe betide the poor sod who has cause to land on Wansted Flats or Greenwich Park, he risks fielding a very heavy book, if you get the drift. And on recent form he'll drop it, heavily.
It could be you!