Glen did something in the magazine a while back. It's hundreds - probably about 1 for every 1000 MGBs you will see!
Yes, don't have the precise details to hand, but about 47,200 built. Of which we have details of around 1300. Some of these will have perished in the intervening years, but a survival rate of around 3% isn't bad! Remeber too that the 47k includes exports too, so you might be looking at only 30/35 thousand UK based. Comepare that to the number of MGB's made & you realise how rare these cars are. G.