H10 is not as congested as you may think. It follows the A40 in with some good greenspace along the south side. Then it follows the west side of the North Circular to Kew Bridge - again with greenspace beneath you. And then you're onto the river - which isn't part of R160 (The Specified Area) so you are permitted to "alight" in the river.
There is no specific rule banning single engine heli flights over R160 - but ALL helicopters have to be able to alight clear of R160 in the event of a power failure.
Look in the EGLL listing of the UK AIP "Aerodromes" website for the full rules.