In summary, here are the RULES
If a firearm was made before 1939 AND
It is muzzle loading OR
It uses an obsolete means of ignition (e.g. pin fire, lip fire, etc) OR
It uses rim fire cartridges (not less than 0.23 inch or 9mm) OR
It is chambered for one of the obsolete cartridges listed in APPENDIX 5
It is Antique and does not need a licence!
If you stick to those guidelines you will not go wrong.
To illustrate these rules, here are some pistols from my collection and the reasons why each is considered to be antique and require no licence.
The second is a rare Colt’s New Service revolver, made in 1905, and chambered for the obsolete centre fire cartridge 44 Russian. Only 5% of the production before 1914 were finished in nickel so this is a particularly choice example.
The third is a Remington Army revolver in 38 rim fire, unusual because it is factory engraved. These revolvers were often converted from percussion revolvers, fitted with a new cylinder and hammer. Typically they were chambered for 38 Rim fire or 46 Rim fire.