The most detailed map in the David Rumsey Map Collection is “London Sheet II.100”, created by the Ordnance Survey Office in 1895 as part of the “London, Five feet to the Mile” series. It’s for the area near Hampstead Heath.
I found it by querying the Rumsey AI Search Assistant for “what is the most detailed map in the collection.” You can ask any question. Is there a map with your name?
The National Library of Scotland has a good collection of the “London, Five feet to the Mile” series.