I see that this debate has in any case been pre-empted by User:Kotniski who has created and populated the ":Category:Constituent countries of the United Kingdom". In my opinion this is unacceptable as 
1) the debate is still in progress, indeed it has barely begun, and
2) the new category name has the rejected term "constituent countries" and is therefore contrary to a decision reached after a long discussion and contradicts the main article's name (Countries of the United Kingdom) and the term found in most other relevant articles. 
Personally, I don't see why Wales et al should not be categoried as 'European countries'. I am certainly against this unilateral reversion to a contentious and rarely used term that has been rejected here. This new category is not acceptable and a consensus decision would also be needed for the proper and logical alternative, i.e. 'Countries of the United Kingdom', if the need for it is proven and accepted.