Looks like M's Office is actually on the "real" MI6 Building, judging by the color of the windows and the architecture.
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Good spot. I did see some crew members standing on the upper balcony of the SIS building; I was surprised they'd been allowed access so presumably it was for this. If they weren't really there on location I suppose the windows would be greenscreens in this shot.
Yes, this is a candid, behind-the-scenes photo; you’d just see the green screen, not the finished composite. To me, this looks very like something that was actually done in the MI6 building – incredible, if that is the case.
Also, the wall with the windows looks to curve behind them, I could be wrong. But judging by the window frame, this could be the office inside the curved section toward the top and centre of the building.