> The problem was that £50 notes were most likely forged, I think, although I don’t know how true that was. So the solution surely isn’t to get rid of the notes as that just shifts the forgery to the next note down. Surely instead the solution would be higher valued notes being created

The £50 notes were perceived as "most likely forged" because - in addition to being temptingly large denominations - they're so rare the average person has never used one and doesn't really know what they look like. There's a vicious circle of course: ATMs don't dispense them and banks are unlikely to unless requested because they're not widely accepted. The £20 note doesn't have that problem.