They’re not allowed on the UK, unless you have a firearms licence (rather than a shotgun licence) and they can’t be used on game. We are limited to 3 rounds. The type of shooting in the video dates back to when guns were muzzle loaded and some people keep re tradition going.
Because this is called double gunning, it’s a traditional form of shooting which dates back to when guns were muzzle loaded and occasionally some people have a day to keep the traditional alive