It's a great example of handling a peaceful exit from a political union, but in terms of maintaining the existence of "The United Kingdom" it remains to be seen. There was no civil war, but it was polarising enough that it could well have lit the fuse that ultimately tears the country apart over time.

Scotland voted against its independence referendum in no small part due to fear-mongering about an independent Scotland being barred from joining the EU. The independence issue was then considered absolutely settled for a generation at least. However post-Brexit that has rightly called this into question, with still strong SNP support Westminster will have to reckon with that soon enough. Irish reunification isn't something I'm too familiar with the intricacies of, but they were pro-EU too and the Irish Sea border doesn't exactly strengthen the unionists case.