Full rules are below, I do get it when people say they are confusing but I think its pretty simple if you've got the time and the will to read the lot (which i did on Saturday night a) because I'm sad and b) Mrs Nobby was watching Strictly. Took over an hour but worth a look if you can be arsed)
I have a divorced step daughter who was worried that her kids wouldn't get to see their Dad on Boxing day as planned so read the rules to clear that up really.
Knowledge is power in these matters and there may be loopholes that allow you to do more than you expected. For example, you are allowed to visit or be visited by someone in your (existing) support bubble (they are still allowed but restrictions do apply) even if they live in a different tier area, they can also stay overnight. Both are
advised against but still legal and in the case of a single elderly parent living alone, this could be invaluable for giving them company at a time when to be alone, would be quite depressing.
https://www.gov.uk/guidance/tier-4-stay-at-home