Why do you think it could be a prototype? Most likely to be an in-service aircraft surely? I'm not sure where those prototypes went after manufacturer testing so I guess it could be one of those, but the odds are against it.
From my (invariably a bit fuzzy) memory, it looks like a photo taken at Lyneham with the dog-leg part of the pan in the background at left and the escarpment to the (east?) in the background at right.
I think your best bet is to pin down a unit and then try to find out which aircraft was 'K'.