In winter you may come across two thorn trees that look similar, the Common Hawthorn and the Blackthorn.
If you look carefully at the thorns, however, you should be able to tell one from the other. The photos shown below are all taken from the Tree Guide UK app.
These are the thorns of the Common Hawthorn. These thorns are usually less than 2.5 cm long. There are no buds on the thorns.
These are the thorns of the Blackthorn. They are much longer than those of the Common Hawthorn. They may be 5cm or more long. There are buds on the thorns.