I am afraid that Regents Park, Maida Vale and West India Quays have all gone up from 5 to 6.
I stay in London a lot, and the hotel utilisation rates last year were exceptional, so the changes probably reflect nothing more than the achieved cash revenue per room.
I agree with your comments on Marble Arch which I have commented on in 2 other posts.
There used to be 3 cat 7's - Grosvener Square, County Hall and Chancery Court. Whilst Mayfair and Grosvenor House might reasonably aspire to the level, I don't think Kensington and Marble Arch are in this league.
It does point to a Cat 8 being introduced at some stage.