Checking this is Blackthorn and not hawthorn in bud. The thorns themselves have lateral buds.
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...have to say, it looks more like Hawthorn, having (as well as the enormous thorn) small, short thorns with a single bud at the base.
If it were Blackthorn, I would expect it to be in flower by now: mind you, you are *pause to check where Pembrokeshire is* a bit further north.
Proof should arrive one way or the other in a few days, when those buds open!
How to take close-ups with cheap phone and hand-lens:
Field Guides for Budding Botanists:
Haven't spotted any blackthorn in flower yet! Seems quite late this year.
For clarity, I should have said that both Hawthorn and Blackthorn have long spines with lateral buds along them.
Hawthorn additionally have short spines on branches, with a single bud at the very base of the spine.
Blackthorn have their buds "in bunches", but it's hard to see this once they start opening - it only works as an ID feature in the depths of winter.
Lat/Lng: 51.7762, -4.7574
OS grid ref: SN098122