They won't. If their argument is "We refuse to comply because we're outside your jurisdiction", the UK will just respond "OK, we're blocking access to your entire site, just like we said we would do to any site that doesn't follow the rules, and in your case we probably should have done that a long-ass time ago. You don't have to follow our rules, and we don't have to let you in." And if their argument is anything else, I can't imagine it holding any water at all. A US-based entity does not get to make demands of foreign governments. At most they can threaten to take their business elsewhere, and that means nothing coming from sites that make no money for anyone and provide no useful services.