Not directly relevant to me but came up in discussion: at least in Scotland you can form an extended household under various conditions, and then be considered as one household for covid purposes. The main situation, which is pretty clear, is if one household has only one adult in it. However another case is "couple who don't live together". Now, this must be there for cases where both members of the couple live in households also containing other adults (otherwise, couples wouldn't need a special case, they could just form an extended household under the main "one household has only one adult in it" rule). What I don't understand and haven't been able to find out is, what precisely is allowed and what is not? It is said that all adults in both households must agree, but what are they agreeing to? For example, are they, or are they not, agreeing to give up any other extended household formation?
Concrete example: consider a student flat containing 5 people. Each person is in a relationship with someone who lives in a different student flat. Do they have to pick just one of them to be the lucky person who gets to form an extended household under the couple rule? Or can all 6 flats (this one, plus the flat of each partner) get combined into one extended household? The latter seems silly, especially when you think about all the other students in those flats and their partners... However, I can't find a clear specification, or even any discussion of it. Has anyone?