Is bullace a hybrid of (probably) damson and sloe, which implies that it could be regenerated, or a distinct variety which came about by a one-time mutation?
I hail from the Welsh Marches and remember my mother mentioning it, but never actually encountered any until living in East Sussex which is similarly damson/sloe country on clay. I find that it's vastly better than sloe for gin: it needs minimal filtration to get something coloured but unmurky, but still has an interesting rustic flavour.
The problem is that there aren't that many, and so far all my attempts at growing from cuttings or stones have been unsuccessful: if they're a hybrid the fruit might be infertile.