FYI everyone. Had this same issue and posted about it tonight but found a simple solution. Just got the flight booked.
Well, that was easy in the end. Found a simple solution. I moved my avios from BA to avios.com. Booked it there for £161. avios.com did not charge anything more than £161, despite all my details (incl UK address) being the same as on the BA site.
So £300 savings out of GRU round trip purely for using avios.com instead of BA's site.