Cannot think of any wild British tree that they could be confused with, there are other plum (such as bullace and damsons) relations, and possibly hybrids, but to my knowledge, none that are harmful. A single tree might be an escaped ornamental of some type but blackthorn ,where it exists, is unlikely to be solitary. As ACE has said, do look out for the 3-4" thorns, they are evil.
Once put one into my knee, it worked as if acupunctured, lost the feeling, and I was felled into the remaining entanglement. Not to be repeated!!