Bullace, damson, sloe, wild plum... to some extent, it doesn't matter much. It is something in the plum family, and therefore not a poisonous berry. Some of these plum fruit don't fit into our nice convenient named categories anyway.
I'm not advocating eating anything on a guessed identification (that would be stupid), but we do all seem to be agreed that it is some sort of plum, so if your interest is eating, pick one and try one. It may be very tart, but try and taste the flavour as well as the bite, since the sharpness can be offset by sugar or other fruit.