M&S always disappoint me at xmas.
party foods rubbish.
Far better to plan ahead and do your own bits .
Does MIL have a signature dish that she can bring along?
For Christmas day itself homemade stuffing and fewer veg but done really well can make all the difference.
We always do a big boozy stew sometime between Xmas and Hogmanay nice homemade soup too to refresh the tastebuds after the excesses.
Lots of nibbly spread teas too cold meats, humous, olives, coronation chicken speciality breads or baked potatoes.Huge amount of cheeses too.
A salmon salad tea always goes down well too. Luckily DH fishes and usually we have some in freezer.
I second good quality chocolates perhaps from a specialist chocolate shop, you need fewer and everyone appreciates them ,I usually make some homemade tablet too to serve with coffees,
I make my own pudding and sherry trifle again superior to anything M&S come up with also buy a sticky toffee pudd from a specialist grocer/deli in town and a couple of tubs of special ice cream from an Italian take out.
Freeze mince pies in advance make them with your DCs.
It's all in the planning .