Are you used to the ILs kitchen, particularly the cooker?
And as someone else said, are you used to doing roast dinners?
If yes to both, it should be straightforward enough.
Smaller joints, boned and rolled or spatchcocked, should cook in that time. Get the meat in as SOON as you arrive in the house.
You could have the veg prepped and parboiled, and either laid out on roasting tins or in large IKEA (other brands are available!) food storage bags (I like Ikea as they are Ziploc, and the large ones are 6l capacity so great for family-sized amounts of veg etc - I often use them to marinate meat or have veg prepped to just tip out onto trays).
Even just prepping the veg and potatoes on 23rd is fine, leaving soaking in water until 25th. Change the water on 24th though (do you have 3 minutes to do that much prep?).
Or parboil and toss in oil/seasonings, and leave in Ziploc bag in fridge on 23rd, leaving even less prep/cooking to do on 25th.
Regular bisto gravy, with any juices from the meat (and a good slug of wine if your family would like that), would be relatively simple to make on the day. Or you could make the gravy in advance, slightly thicker than you'd like, store in fridge and add juices when reheating.
Pudding could be microwaved, or even steamed if you have a spare ring available, in that time frame (don't forget, you get the extra half hour or more while you eat starter/main for that to finish).
Do you want a starter? A pot of soup might be nice and something that could be done in advance, with part baked bread rolls to throw into the oven when you turn up the heat to get roasties crisp as the meat comes out a few minutes before serving.
Or smoked salmon with brown bread and some cherry tomatoes and lemon wedges - get ready-sliced salmon.
Or make the filling ahead of time to reheat on the stove for vol-au-vents, and buy frozen vol-au-vent shells, to pop into the oven when the meat comes out.
Or just some nice M&S canapes/party food, either to serve cold or to throw in for the last few minutes (again - when the meat comes out to rest). Maybe a few cold nibbles might be handy to have anyway to take some pressure off the cook.
In terms of kitchen stuff, does MIL have the right number and size of pots for your plans, enough knives of the right size and level of sharpness, peeler, oven gloves, whisk, carving set, serving dishes and cutlery, wooden spoons, jcloths, tea towels, aprons, roasting tins (large enough for a family joint, or more than 1 for veg etc as well) and other baking trays (part baked bread or canapes or dessert items?) etc? Does she keep rolls of kitchen towel, tin foil, cling film that you can use, or should you bring these bits with you?
What seasonings or other ingredients that you regularly use (and would likely use on the day) does MIL either not have or not use regularly (so use-by dates may be, um, suspect?), or you are more comfortable with your own dispensers - that you might want to bring from home? Salt and pepper, olive oil, gravy powder, any herbs or spices you want, cup of brandy for the pudding, .....
Think through your menu in general, and then lay it out in detail to see exactly what ingredients you need, and whether you need them at home for prepping, or at MILs for cooking. And also the tools etc you'll need.
Then do your lists, including a master list of everything you need to bring with you. That should make it easier to pack everything up ahead of time.
If you are bringing cold stuff, do you want ice blocks from the freezer for a cool bag?
Does MIL normally have ice for drinks, should you make some in those "freezer bags" for making ice, to bring with you, or buy a bag and have that in your own freezer to bring?
Can you have anything in her house ahead of time - whether ingredients or utensils? Does she have freezer space to put a few things into in the days/weeks beforehand (like par boiled potatoes, ice cream, ice and sliced lemons for drinks, all sorts)?
Or is it a case of literally bringing everything yourselves on the day?
(The sliced lemons is a trick I learned a while back - we only ever tend to use 2 so wasted loads, so now I slice a whole lemon, freeze any spare slices flat on a tray, bag them up once frozen, and when needed, throw them straight from the freezer into drinks where they are perfect - and saves an icecube as well!).