What's probably blackthorn at this time of year.
I dare say.
In a hedge, thorny, and no leaves to help with the ID
Blackthorn normally flowers before the leaves appear.
There is a similar prunus, cherry plum, that usually flowers a bit before blackthorn. The shrubs look like blackthorn in early spring - drifts of white blossom covering the whole shrub/tree Leaves appear while they're still blossoming, unlike blackthorn. I don't think they're particularly thorny, unlike blackthorn; we have them locally, but rarelyusually on verges of roads where I would not feel comfortable about stopping to look at details. It's not native & IMLE tends to be commoner near urban areas.
My guess is also blackthorn, which, according to the books, has black bark rather than brown, though I've failed to separate bullace from blackthorn at this time of year. FWIW bullace is another non-native which has escaped from cultivation & has bigger fruit. There's more than one variety; some have round fruit while others are plum-shaped.
I think blackthorn is the only one which has a high (>50%) probablility of minor wounds from injuries caused by the thorns turning septic, but I'll leave controlled experiments to others.