Suspend disbelief for a moment and imagine the following scenario.
Following an ultra hard Brexit, with Irish border issues, remain leaning Scotland and northern Ireland leave the UK and enter into another loose union with Ireland, to make use of their EU membership. Let's call it the Celtic Federation.

The Celtic Federation extends an invitation to Wales to join, and there's a referendum in Wales on whether to

A) remain in Britain- which is now basically England and Wales, out of the EU.

B) join the Celtic Federation, in the EU, Schengen, uses the euro