I never order anything. I just go to the shops first thing on Christmas Eve (or Dec 23rd, but not this year, because that's a Sunday and that's usually too busy) and buy stuff. Get there between 6 and 7 am if you can, and you'll be fine.
Just have a plan B in case your first choice isn't available, but there will always be turkeys, sprouts, potatoes, stuffing, cream, cheese, desserts, etc etc in enormous quantities in every supermarket in the land (unless there is snow, which is forecast in some areas of the country) so there's really nothing to worry about.
I will probably go to our local Morrisons, which is in a town with a butchers, market and veg shop, so will probably try and get everything in Morrisons, but if there aren't any sprouts or the spuds look a bit rubbish, I'll go to veg shop. We usually have a turkey butterfly (single large turkey breast) which I'll probably go to the farm shop for and they always have a load in the fridge that you can just buy without ordering.
For cooking, I have a spreadsheet of timings that I've honed over the years, so cooking is fairly stress free - it's just a slightly elaborate roast dinner after all, and don't feel you have to do all the sides and veg you can think of.
Also ask your DM and MIL for advice and run through your shopping list with them to make sure you haven't forgotten anything, as it's likely they've both cooked dozens of Christmas dinners