Realistically they can't, unless the service owner lives in a country with an extradition treaty with the UK, and that country has an equivalent law in place. B

Realistically they can't, unless the service owner lives in a country with an extradition treaty with the UK, and that country has an equivalent law in place. But most service operators don't want to deal with that stress, so they will just IP block the UK and Brits who know how to use a VPN will just keep using the service. I wouldn't be surprised if this ends up being a topic in trade negotiations with the US in the future though, since this is a trade barrier that imposes significant regulatory cost on US companies for content that is legal in the US. Eg. the proscribed categories of illegal content include knives and firearms, hate, etc.