It's hard to quantify who is best Mistress in London as I've only visited one Mistress in the past 11 years and anyway it is very subjective but Mistress Lubyanka in my opinion is as good, if not better than ever!
As for the premises they are up to her usual impeccable standard. I know you do not like Croydon but her premises are very discreet and only a 10 minute walk from either West or East Croydon Stations

I think my session with Mistress Lubyanka in May (my first with her for at least 3 years, alas, alas) was perhaps the best session I have had with her, certainly letter than any since the time she used to use Mistress Avalon's premises, and perhaps because she always felt the need to exercise caution in her previous place, great house and location though it was. Excellent though she remains, I can assert however, that Mistress Lubyanka is not the best mistress in London. But only on the technicality that as far as I am concerned Croydon is in Surrey and not in London, whatever the London Government Act 1963 might have to say about the matter. :D

For a moment Petronius my friend. I thought you were going to get yourself into trouble.:D