My best friend is dairy free for medical reasons and she was my lockdown buddy, so this is what I did last year:
For a Christmas Day starter, replace cream cheese in smoked salmon starter with capers, chopped dill and a lemon wedge. Or have soup, loads of them are dairy free if you don’t want to make your own.
The main course is easy to make dairy free. I would treat yourself to the tastiest meat you can afford and you won’t miss dairy at all! But If you normally make leeks or cauliflower in cheese sauce as a side, the sauce in This recipe is nice, although not an exact replica of cheese.
Check labels if you are buying a Christmas pudding, I think a basic Asda one was ok last year when cheered up with a soak of extra brandy and had palm oil as the fat, but the supermarkets normally sell a number of different types and they do change recipes. If you normally serve brandy or rum sauce, make a basic white sauce with dairy free margarine or vegan block and barista oat milk, then add sugar and alcohol. There are loads of nice vegan ice creams now as well. We love Swedish glacé.
Oatly cream is a tasty whipped / clotted cream substitute. I haven’t tried it to see if you can blend it with alcohol without it separating, but you could tip some alcohol over the pudding then add the oat cream on top? I reckon you could also make a cream tea or top a trifle with this cream (and decorate with berries and grated dark chocolate). Boudoir biscuits are generally dairy free if you make a trifle.
For making cakes or pastry (for mince pies and quiche), vegan block is good if the recipe calls for butter. Trex also works. If you can’t be faffed with making your own pastry, Jusroll ones are mainly dairy free.
For snacks we had a luxury tray of nuts (think M&S), and loads of the posher crisps and lentil chips are dairy free, as are twiglets. Booja booja chocolate is fab as others have said. If that’s too pricey, just go for supermarket dark chocolate, a lot of that is dairy free (but not all). Some of the Ritter sport chocolate flavours are dairy free but I’m afraid I can’t remember which!
One tip I’d have is if you can afford it, place an online order for extra bits with a specialist store. I used Ethical Superstore last year for picking up Booja booja chocolate, dairy free shortbread (nicer than butter shortbread!), crackers and all sorts of other goodies.