I know if I was getting work done to the frame, I'd have the frame repainted at the same time. You've mentioned Ellis Briggs. I've had work on frames done by them, and one respray.
The paintwork doesn't match their quality of the mid eighties. Tempted once or twice, but working where I could have the frame cleaned & painted, for nothing, meant I chose the cheaper option.
I'd rule out the builder/worker number. They'd have their own set of dies. Pass the frame onto someone else, how would they have access to the other persons tools(no chance of borrowing them) or know who built it.
Any numbers/marks on the inside of the rear forks?