Several people have pointed out the futility of trying to apply your country's laws to people based out of other countries, but it isn't quite as pointless as you might think. There are all sorts of mechanisms for handling this, starting with extradition treaties (where the whole idea is countries selectively respecting each other's laws), as well as working directly with banks. The US is not the only country that pulls the 'well, because your bank wants to operate in the US, they accept orders to freeze your funds'. The UK is, after all, another major banking hub that can exert that kind of pressure.
Now, the politics of extradition tends to be pretty expensive, so the UK would have to weigh if trying to go that route is really worth it, but ultimately all they have to do is convince US officials to go along with it.
Or they could outright kidnap them. That president is pretty well established by the US, it would actually be kinda funny to see some country turn it around.