This is one of those technically true but defacto false things. Its legal under UK law, but if they want the variety of benefits they get from the US then it isn't. If the UK government starts arresting vacationing Americans for things that aren't a violation of US law, its all a matter of if the US governments wants to make an issue of it. Maybe you get lucky and nothing happens, or maybe you lose your military protection and 25% of your GDP. Plus your tourism businesses take a hit. You really want to take that risk?

But given the behavior of the UK government lately, doing something suicidally stupid seems on brand for them.