The UK seems fairly relaxed about constituent parts leaving (Scotland was given a referendum, NI has option to leave if they want) - which I don't believe applies to US states?

Scotland's referendum, and its parliament, were gifted by the UK Gov, they don't have any unilateral rights.

US states can't secede unilaterally either (any more!), but at least they have a right to their own leadership and militia to do something about it.