We like cooking from scratch (and prep a lot of it on Christmas Eve as a family - DD may slope off to watch tv partway through, but it is part of our "start of Christmas at home" routine).
But we do buy a lot from M&S on Christmas Eve. Some bits for a smorgasbord-type dinner of cooked and cured meats, prawns and smoked salmon, flatbreads, olives, cherry tomatoes, cheese etc - all nice picky bits that we love.
A birthday cake for DD (26th Dec).
And loads of party food bits for over the period - we do at least 1 if not 2 packs of things on Christmas afternoon (while opening presents and waiting for the turkey to cook), and then we have an "at home" type gathering for neighbours and extended family on 26th so there are people in and out all afternoon and a few different packs to cook quickly at different times is very handy. Mostly savoury, but some sweet/dessert things too. And a few large sharing bags of various crisps.
We also now buy a half bottle of port, as it is nice port and we wouldn't finish a whole one between just 2 of us over the season. But a glass or 2 with cheese a few nights over Christmas week is lovely.
So think about what you as a family would love most.
If you really fancy the idea of a "throw it in the oven" dinner, great.
But there is so much lovely stuff, that you could get a lovely beef wellington or duck or something and sides for another dinner (and do your traditional turkey dinner for Christmas as normal).
You could get all your nice snacky bits for various visitors or gatherings or whatever.
You could get a load of nice wines or port or champagne/prosecco.
You could make up lovely hampers as gifts.
Or any combination of those. So maybe a few sides for Christmas Dinner, a nice main course for another easy cook dinner another day, some nice wine, and plenty of nice nibbles to fill in the edges.