Resolves NA if the UK has not joined either by the end of 2050

Resolves to the year that the UK officially joins either of the two, not when such a decision is taken.

In both cases, the determinant of whether this has taken place will be the Wikipedia entry.

https://en.wikipedia.org/wiki/European_single_market

Wikipedia allows for the fact that non-EU countries in the Single Market may have certain exceptions or carve outs.

https://en.wikipedia.org/wiki/European_Union_Customs_Union

There are non-EU countries that are in the EU Customs Union (like Monaco) and there are non-EU countries that are in a customs Union with the EU (like Turkey. This questions takes the definitive view that the later case 'counts'.