Not all groupings are based on trade. The 5 eyes is based on intelligence for example, and only involves the UK and no other country in the EU. Because the two do not compete.

Barring sanctions, there are few cases where one is forbidden to trade with third parties - not even China prevents trade with Taiwan.

Given that the situation is not the Axis / Allies in WW2, countries can have relationships with both and doubtlessly different relationships with both.

If any grouping were to be created it would be extremely unlikely to be a trade block - who said it would be? If there was a sharing of the UN Veto this has nothing to do with it, since again France and the UK have one and the rest of the EU does not since the two are not linked.

And holding back can be semantics. Is a trade deal with Canada almost destroyed after 11 years due to a province of Belgium a good method? Will this be the same in the next trade negotiation? Will countries use the EU blue print, or will countries just not bother with the UK?