I hadn't noticed that in amongst all of the announcements on tiers and Christmas bubbles, Johnson also sneaked through this change on support bubbles:
So as long as a household has a child under the age of one in it (regardless of how many adults/other children are also there), that household can now join with another household, of any size, in a single support bubble. These combined households will only count as a single household for the purposes of the "three household" Christmas bubble plan, which will obviously benefit Boris and Carrie directly (how nice for them!).
A friend from work is planning to make maximum use of this change. He, his wife and their nine month old are currently living with his in-laws. His brother, his wife and their two kids are living with her parents. From 2 December, the two households can bubble up. That's eight adults and three kids, which will only count as a single household for the purposes of Christmas. Collectively, they can start searching for the two other households they want to incorporate into their Christmas bubbles for the 23-27 December period.
If my friend wanted to really push the envelope, he might even choose to form a separate Christmas bubble from his in-laws, even while he continues to live with them:
So you are free to be included in a different three household bubble to the people you are currently living with, even while you continue to live with them. Both my friend and his in-laws would therefore be free to bubble with two other households each (all the while living in the same house and with him still being in a support bubble with his brother, sister-in-law, her parents and his kids via the support bubble change on 2 December). That is now probably 30+ adults and god knows how many kids, frequenting maybe one/two houses continuously for a period of five days - not a single rule would be broken, you'd just be advised to clean the door handles a bit. It's nonsensical!