Depends what you mean by London. City of London or Greater London? Lot of find spots all over Greater London and a trip to the brilliant but "No photography" London Museum will reveal much. So many flint tools and deliberately bent Iron Age swords and skulls found in the Thames, especially near Battersea, that it has been suggested that the Thames was an Iron Age sacred (that word again) river - a British Ganges. Don't believe the myth that there was nothing there before the Romans.

I grew up near Stoke Newington common unaware that a very significant Palaeolithic site was beneath my feet. The Hackney Brook is underground there too - but no one would ever know.

Megalithic London? Its there, but hidden and largely un-photographable.