12 is a lot of people, I suppose multigenerational so different diets and preferences.
Things you can prepare in advance:
2 large lasagne, one traditional and one vegetarian (spinach and ricotta, I make asparagus or pesto)
A gammon to serve cold. Salmon if you’re fancy. Cheese and crackers.
A variety of salads. I like shredded fennel, carrot and celery salad (sort of coleslaw without mayo).
A sandwich platter for kids. Chocolate rice Krispy nests with mini eggs.
Cake and/or dessert. I’ve never had Simnel cake as we’re not fans of marzipan. I usually get the kids to make tiramisu the day before (raw egg alert) but this year it’s Summer pudding with strawberries and frozen mixed berries. It has to be made in advance, is always a hit, and is vegetarian friendly. Serve with or without pouring cream.
So you only have to prep the veg and roast (I’d do 2 large chickens, depending on budget).
Aunt Bessie’s Yorkshire puddings are fine.