I was mulling over possibilities whilst reading your post, and came to the same conclusion as you by the point you did
In the UK, if there wasn't a specific CP tick box, then Married would be the one to choose, as they are legally one and the same (AFAIK).
Here you can't tick what is not recognised. Hence as you suggest that leaves you in the eyes of the authorities here as single. ['''You live with a friend'''.]
p.s. Of course that status could be adapted according to any benefits your employer/s might accord spouses in a CP. i.e. to the State you're single, to your employer you're in a CP and your spouse is (potentially) entitled to a spouses benefits.