It does say in the competition rules for the 2014 series :
The BFTA reserve the right to vary the 7 nominated events at any time
I can't find anywhere in the constitution, bylaws, meeting minutes or the competition rules etc that it's 1 GP per region. That might be implied from the allocation of GP's but it doesn't seem to have been recorded as a rule.
So with that in mind, I'd say that since the BFTA has the right to vary the events, it would be within the constitution to have GP6 rescheduled and even shot in a different region.