If Northern Ireland leaves, I guess the UK no longer exists, and we switch to ".gb". Could clone .uk at .gb for a while (ten years?) and give people time to switch, but seems more likely Nominet would prefer to just charge people twice for a .uk and a .gb.
If Scotland and Northern Ireland leaves, that leaves us as just "Britain". Maybe we create a new ".brit" (or whever the 2 char country code will be), and give time for .uk to disappear again.
Wales wont leave.
[edit] I just saw emmelaich's comment that .gb already exists.