If you have a veggie alternative (I usually do a chestnut & mushroom filo pie), make enough for everyone, or the poor lone actual vegetarian will be fighting the rest of the table for it.
Freeze parboiled potatoes, parsnips & other root veg a few days ahead - tipping them frozen into hot fat crisps them up. Also works for stuffing balls.
As everyone else said, the bird/joint will stay hot for hours under tinfoil & a towel, & be better for it, so do that first then the accompaniments.
Better to cook 3 vegetables perfectly than have 6 that are variously half raw, soggy, & cold. I do carrots, sprouts & peas (not traditional but the anti sprout brigade like them) in a hob steamer. Red cabbage is slow cooked ahead with red wine & spices, & then can just be heated through in the oven or microwave.
Loads of gravy & make sure that's one thing that's piping hot. Other condiments from jars/packets unless you especially love making your grandma's bread sauce recipe from scratch for nostalgia purposes.
Do canapes & drinks in lieu of a starter, then offer puddings buffet style an hour after dinner (when someone else has done the washing up, basically...).