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?
Relaxed is not the word at all. Northern Ireland is a powderkeg and the UK government has known that for decades.
The Scottish referendum did not use neutral international observers, and had extremely biased coverage by state media. The BBC itself even tried to stir up forms of violent unionism. (Look up the so called "passionate Highlander", a drunken loyalist type who talked about defending the union to the death "in the name of Jesus Christ".)
The Scottish referendum was gifted by the UK, as was its parliament which can theoretically be dissolved by the UK at any time.
The NI thing is different, involving a peace treaty between the UK and Ireland.