Wellington Square is not a true English Square - it has no park in the middle of it. And 30 Wellington does not possess a Bay window. I would instead nominate 36 Markham Square, on the north side of the Kings Road. Also, perhaps it is no coincidence that Kingsley Amis was given the name Markham for his nom de plume. The offices of the Secret Service are contained in a nine story building overlooking Regents Park in London.
Some stories do place the height at eight stories.
Of the interior, We know that Q branch has at least part of the basement for us as a shooting range. Records is located on the first floor, and the canteen is around the second floor and serves steak and kidney pie on Wednesday's. Bond's office is on the fifth floor. He shares it with two other agents. His office looks out over the Park.