52 Radnor Walk: Quorum - December, 1968.

Tammy posted this over at the ever excellent Beatle Photo Blog, suggesting it might be the dressing room at Kenwood. That was my first thought too upon first glance, I must confess. But there is clearly an end terraced house outside the window, so unfortunately not.
The pic is from the March 18, 1969 issue of Look magazine, and captioned "Lennon and Ono boutique-it-up on King's Road". My guess is that it was actually taken in Quorum, which occupied a house at 52 Radnor Walk (just off the aforementioned "King's Road"). (The exterior "then & nows" above seem to match.) It's mentioned in The Beatles' London as the subject of an amusing quote from proprietress Alice Pollock, concerning a 1969 visit from the pair: "Yoko had a skirt shortened that was so short anyway there was no skirt left. John just went around growling at everything."
The mag (Look) is well worth tracking down, for a verbatim slice-o'-life at Kenwood and elsewhere in December, 1968. Though it is fairly anti-Yoko, it provides a few interesting quotes, including some bickering about their initial meeting at Indica, during which John claims not to remember meeting Yoko at the gallery at all. (He may, however, just have been saying this to irritate her, as she had annoyed him by claiming he "looked so ordinary" at their first encounter.)
"You're always changing the story," quoth Yoko, "I'm getting uptight." "What're you getting uptight about?" ripostes John, "Because I didn't remember you at the Indica Gallery? Serves you right. After sayin' I looked just like a bank clerk."
Ahh... such squabbling twas ever the stuff of young love.