Well, let's be rational about it, shall we ?
I too think that they look right sized for sloes, but....
Is there anything else at this season that is of that size ? and colour ? and shape ?
The only black berries around that I know of are,
Black rosehips, and those are bigger and round.
Damsons, which to be honest are well past here, are a bit bigger and are inclined to be purpley black.
Tutsan, which are smaller, and tbh, I've never seen it growing here. Though Culpepper knew of it, so maybe
Bilberries, which if they are that size will be soft and kind of squishy. Definite belly button end where the flower was.
Elderberries, which are much smaller and mostly all gone to birdfood now or shrivelled like currants.
Ivy berries, which are rounder and aren't in black fruit yet.
Black nightshade, again, rounder, smaller, and it grows in clusters.
Deadly nightshade is about a cm in diameter, doesn't have the single stone of the sloe and it comes in a leafy cup.
Laurel...cherry or bay. Grow from racemes like wild cherries; small with almost pointed ends.
I don't know of any others.
I reckon they're sloes.
cheers,
Toddy