They are muzzle loaders.
When the ban came in gov said we could keep muzzle loaders as no one had commited any crime with them and the technology was obsolete. They thought a muzzle loaded pistol was just a single shot piece as many use for historical displays, of course we know different.
There has even been a modern design in the Patriot that uses front stuffing blocks.
The revolvers raise a few eyebrows when visitors go to ranges and along with the longbarrel pistols show what an ass the law is.
“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