Get two extra washing up bowls and put 5 dinner plates in each. Fill them with boiling water. Put a tight tin foil lid on each and set aside. They'll stay very hot for about 45 mins to an hour in another room (safe from little prying fingers who we do not want to get burnt)
Hot plates go a long way to keeping food hot.
Make your gravy into a large teapot. And I wholeheartedly advocate the instant kind. Not faffing about with pan juices - they just go cold too quickly. Make it and put a cosy on it and again put it out of the kitchen away from little fingers. It's my best Christmas advice. Hot gravy really helps a meal. And a teapot is a lot safer than a gravy boat or a random jug. Everyone will nick this idea because it's brilliant. Trademark Crucible....
When a vegetable is cooked, strain off the liquid into a strainer and put the strainer over the hot pot it came from, over the turned off gas hob, which should still have residual heat. Put a smaller plate (probably a side plate size will fit) over the top of the strainer) and you'll have them steamy and still warm for longer. Trap the steam!
Beware of extractor fans and open doors. Yes you need air and people need to move about but don't lose ambient cooking heat in a smaller kitchen.
Peas are hard to keep warm. Make them last of all, probably when food is being served, and line the serving dish with tin foil.
Most of all, wrap the joint in clean fully towels, and make sure it's on a towel too. A cold surface can rapidly cool a turkey dish. Do not cook it in a big foil tray, the ones that get flogged everywhere at this time of year to save washing up. They're an absolute safety nightmare for a big joint. Get a proper dish to roast in. It will stay hotter. Charity shops usually have something.
Good luck. You'll be grand X Merry Christmas.