not sure. I guess any frame number will match the sequence of whoever built it - so you might be able to cross reference it with, say, Holdsworthy's number system. But if they sourced from a number of builders that might prove tricky. Plus, as GB were simply badging up other peoples frames they probably wanted to keep the frames true origins vague.
I was just reading about late '80s/90s FW Evans frames and saw this that might be relevant:
Their own brand frames were then farmed out to Orbit but quality control issues meant that they later transferred production to Nigel Dean. Orbit and Dean supplied own brand bikes for several London shops, it wasn't an unusual arrangement. Dave Yates made frames for Condor after Dean closed.
so that could be two other possibilities