The 4 nations Christmas plan is a total minefield.
It will be fine (limited households) to go visiting, eating and drinking.
The travel restrictions will no longer be law for anybody by the looks of it.
Regarding tier 3 and 4, using that days to go solo or social distanced hillwalking or other outdoorsy stuff instead of visiting, eating, drinking - not sure about that.
Looks like the guidance / advice will still be that's not acceptable but for that short window it will no longer be law.