It slightly annoys me that I keep hearing about the ‘3 household’ Christmas rules. The law is not as strict as that - since each household can bring a ‘linked household’ the real limit for many will be six households. A very different proposition

And that’s not all...

...In the last set of rules the number of households which could be a ‘linked household’ was significantly increased to include eg households with (only) a child under one

AND...

... the newest rules also allow for a household with one adult (the original definition of a linked household) to remain a linked household even if an adult student came back to live with them

AND...

... a child or children who live between two households can move between them during the Christmas period...

So a lot wider than people assume - and lots of potential for making stricter without getting away from the ‘3 households can mix’ which people understand to be the case.

AND

Linked Christmas Households can be made up of individuals, not households

So
Member of Household (HH) A can link with
Member of HH B &
Member of HH C

But during Christmas period go back to original households who can be linked to other Linked Christmas Households!

For those calling this "Tier 0" I think that is actually pretty accurate as the reality of the rules is that they can lead to almost unlimited indoors mixing between households.