We hold antiques under section 58 of our firearms act.
Interestingly as well as obsolete calibre it also says all muzzle loaded firearms are sec 58, no FAC needed. Our HomeOffice issues guidelines to local licencing offices that modern replica m/l's must be held under section 1 and on a FAC. But they are guidelines our Law says different.
The hundred years I think covers all antiques or old stuff rather than our firearms that can be held. Not looking at the Firearms act again today but 2 dates flash before my brain cell of 1919 and 1939, we could certainly hold a Colt 1911 in an obsolete caliber.
As well as obsolete caliber we can have rare or special interest or forming part of a collection firearms, like someone I know has a collection of all the AMT stuff stashed around the world who would like to have it over here. Not particulary old but no longer produced so potentialy of future historic interest.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane