I will next time I get a broken nipple on my one and only BP revolver. I have to admit that I've never seen a broken one yet, but there's time, I 'spose.
I amended my initial post for content. With a broken stud/bolt/screw you drill a suitable-sized hole in it [remember that a nipple already has one] insert the appropriate-sized stud remover - a left-handed thread item - and simply unscrew in the usual way. Harbor Freight sells a set of useful sizes for less than ten $$$.
tac
PS - the guy looking over my shoulder says B&Q do 'em, just like anywhere else. Halfords? Got no pound sign on my PC, hence use of $$$.