This is a guess: I thought this could be a stretch of roman paving / cobblestones. Since old roman roads are referred to as 'watling roads' I'll go for 'Watling Street', a little east from St Paul's cathedral?

I can see the rationale but no. Not Roman and also not stone...
You might get a further clue if you look at the silver birches also in the pool.

Indeed 'tis, Jim. Well done. Fascinating that it's the only wood paved street in London. There were apparently quite a lot before the war. The ones that got damaged by bombing apparently made good firewood...
(The pool is diminishing rapidly. Will have to find replenishment...)

It would have been a quieter surface but I think also it was safer as horses were less likely to slip. Drawback was it was difficult to clean and absorbed a lot of dirt, including oil once there was motorised transport. That, of course, increased its effectiveness as firewood!
Fascinating area round there. I've been asked to do a guided walk for a conference in October and this will be included in it.
(Interesting about Kilburn High Rd. Wonder how much more of this there is that's covered up by tarmac?)

Well done Jim! This is absolutely fascinating... and quite scary to imagine what could have happened if a fire had started...

London wooden block paving: discuss!
Yet another thing that warrants further investigation...