You do have time to book a supermarket delivery slot, just pop a couple of items in your basket to make up to the minimum spend, then add what you have decided on later.
I wouldn't do a traditional Christmas dinner, especially as you have never cooked one before. I'd also get a lot of pre prepared items to cut down on your prep time as much as possible.
I'd do a large vegetable stew with tinned lentils and haricot beans, using packs of prepared root vegetables, a large dish of Boulangere potatoes using vegetable stock such as Marigold, a large beef casserole.
Your guests can then select what they want to eat from the dishes, with something for everybody, with the vegetable stew being a side dish for the meat eaters.
For pudding I'd make a winter friut salad, poached but served cold, dried fruit like apricots and figs, fresh apples oranges and pears, and some Christmassy spices. If you felt you needed a starter, avocado vinaigrette would suit everyone.
The pudding can be done a couple of days beforehand, timing isn't critiacal for any of the dishes, so no worries there.