[hirundo]

It's complicated. See the Edinburgh Agreement of 1992:

https://en.wikipedia.org/wiki/Edinburgh_Agreement_(1992)

Given that it's not unreasonable to see Denmark as an "active" but not "full" EU member.

[pjc50]

By those metrics the UK isn't a full member either, since we have derivations - and Denmark is in Schengen!

[ThomPete]

That's the point.

If we were full members we would have the EURO and no derivations.

The fact that the UK has kept their own currency is itself making it less of a headache to untangle from the EU.