I have posted an article about the conditions that an old gun must satisfy to be considered an antique in the UK.
The law doesn’t define it but there is guidance that does. Ultimately it is up to a court to decide whether or not, taking all the facts of the case into consideration, whether a particular gun is antique.
However if a collector sticks to the Home Office guidance then he or she will never be prosecuted.
Some police licensing departments have accepted guns as antique that are not covered by the guidance. But’s that an advanced topic for another time!