Hypothetically if the UK (or at least what was left of it) were to become the 51st State of the USA, we would be subject to US federal law. But can anyone think of any US federal laws that wouldn't work in the UK? Here's a few I can think of: Having to drive on the right (which, ironically, I suspect some Brexit supporters feared Brussels would make us do sooner or later if we had stayed in the EU!). Having to adopt US (not UK) imperial weights and measures, which are different from those used in the UK in some cases. Having to lower our speed limits on motorways (or highways as the Americans call them), which I think used to be 55 mph everywhere in the US (I think it now varies with the State but is still generally lower than in the UK). Having to raise our drinking age to 21 or take a cut in highway funds (technically the drinking age is a State matter rather than a federal matter, but in practice there has been a federal law in place for about the last 35 years that says any State that doesn't have a minimum drinking age of 21 has to take a cut in highway funds). If we had to do that, a lot of pubs whose main clientele are 18-25 year olds would struggle to survive, and in university bars it would be a nightmare stopping students under 21 from going in (or making sure that they only have soft drinks).