There is or was an approved area for seaplane landings in the Greenwich area. Perhaps this should be investigated. I wonder if landing a helicopter on a barge in a designated seaplane landing area requires planning permission! would it still count as a landing on water?
Really? I've tried to find anything about Thames landings and come up with zero.
But I doubt London CTR would allow the approach. And even if they did, there's an 8kts speed limit on the Thames, I think. One could perhaps land a seaplane more to the east on the river and speed taxi into suitable mooring, but I'm sure there's some law against that too.
They love laws.