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I don't live in London anymore, so my perspective has shifted quite a lot. To be honest I have some amazing memories of riding in London, but it's not really the city where you can have a relaxed ride enjoying the skyline, the architecture and the life around you. Exceptions apply, of course. The Thames path, as mentioned by @skydancer, was a favourite (Hampton Court to Richmond usually in my case).
Are you going on holidays or staying for a bit? If it's for the holidays I would forget about the cycling bits, it's a bit like going to Las Vegas and ask which libraries you should visit.