I totally agree with the OP in questioning HR on how they are squaring this out with the current government guidance. Unless they are proposing to meet up between 23 and 27 December which seems extremely unlikely for a corporate event?? Even then, only people from 3 separate families can meet together indoors. That would limit the event to 3 people in total, surely?
In my own experience, back in July, my company was registering us for an in-person industry conference scheduled for October involving over 1,000 participants, while it was very obvious for any thinking person (but of course not anyone named Dido) that the infection rates were going to explode in autumn.
So I went back to the sender copying everyone on the original email and the FD who is my manager, and giving an example of the recent NHS trust training event, and proposing that we go back to conference organiser asking for a virtual attendance options. You do not get things changed or improved for yourselves or your families if you keep your heads down, ladies! I am really surprised most of you advise OP to do just that and enable idiotic (and in this case, illegal) actions.
I had an immediate response from a couple of shielding colleagues supporting my proposal, and so the FD PA who was organising attendance of the event went back to the conference organiser, and 2 weeks later the conference was transformed into a fully online event. I suspect lots of other participating organisations had similar requests.
So OP if you feel strong about it, I would keep pressing the HR but with a reasoning - e.g. you are genuinely worried about the health of the colleagues, protecting NHS, and the potential effect on the company's future etc., plus the clear illegality of the proposed party, and this is very black and white IMHO.