The Seed Library mentioned is actually quite a good bar, but in no way it is remotely luxury.

The Connaught bar remains at the top of london hotel bars, and indeed of all cocktail bars globally. In terms of “newcomers” (I.e places less than a decade old), Donovan’s at the under-rated Browns is probably the place to go, or possibly Velvet at the rather overdone Corinthia (helmed by the same famous Italian bartender as Donovan’s).

I’d also agree that Silverleaf is worth a visit if passing by that part of London (it saves one from the Andaz’s poor bars opposite the road), but don’t think it’s the best cocktail bar of the area (ref: Happiness Forgets), nor is it comparable to the higher end bars centrally. It can at times be more personable though.