Here's an interesting photo that I'd not seen before today. Obviously 'A' is in the middle, but I don't recognise the other two. I am tempted to say that I think the picture must have been taken down near the embankment on the north side of the Thames and that beyond the cranes in the background, we can see Big Ben. That would be my guess anyway.
I'll guess that the other two girls are models, although I had briefly wondered if the blonde one was Rosemary Donnelly, but I don't think she is. Probably a promo shoot for either the series itself or the fashions.
Regarding this photo, I found 4 others on MirrorPix, all appearing to be taken on the same day. Doing some more research, these were taken in Birmingham on 15 September 1970, for a photo to appear in the 16 September issue of The Birmingham Post. UFO premiered in parts of the UK on this date. Here is what appeared in the newspaper (which I also found online)