13ReadyMeals
I think (though not 100% sure) that existing bubble are replaced by the xmas bubble. Otherwise you could end up with 6 households all meeting up if they all took their existing support bubble with them! Also it wouldn't be a very popular rule clause because that would mean the person you'd bubbled with year round - for example two single mothers and their kids, would have to choose one single mother's family to stay with over xmas even and stay there together with some family they'd probably never met before and not see their own mother and nan! No I think the existing bubbles are set aside over the 5 xmas days
This is what the Gov.UK site says, and I interpret it to mean that the existing support bubble remains in place, counting as one household, and can join with two other households.
1.1 If you’re in a support bubble
Existing support bubbles count as one household towards the three household limit. This means that if you are in a support bubble, you can collectively form a Christmas bubble with two other households.
I'm not saying that it is necessarily a good idea for everyone do this, and agree with your comments on how it could mean six households could get together, which sounds crazy, but it is what is 'allowed', though the powers that be have added the following words of caution:
This applies only to support bubbles as set out in law. You should, however, consider the risks of doing so and keep your Christmas bubble as small as possible.
Childcare bubbles are a different matter altogether, as they only permit childcare, not socializing, so play no part in the Christmas bubble arrangements. (I know you didn't mention those, ReadyMeals - I'm only mentioning it because I've seen them referred to in a few other posts)