Agree.
I would order the following:
a selection of sliced meats (serve cold, can you get them ready done on the platter?)
some 'posh' salad - coleslaw, Waitrose beetroot (if your budget stretches) is yum, potato salad
normal salad - chop up tomatoes, cucumber, pepper sticks, carrot sticks
ready to go in the oven: stuffing balls; pigs in blankets; sausage rolls; quiche
Optional: nice cheese, pate, crisps/nuts (to nibble whilst everything comes out of the oven)
Plus oven-heat bread (the ready rolls that take ten minutes) in case you can't get out for fresh bread on Boxing Day.
Side options: butter, pickles, pickled onions, gherkins (just leave the kars on the table!)
Pudding: mince pies, order a trifle or chocolate cake. Job done.
Supermarkets have offers on Christmassy disposable plates, napkins and tablecloths at the moment.
Could you also ask people to bring and share leftovers? We invited a few 10 friends over for Boxing Day last year and they brought leftovers even though we hadn't asked as they couldn't eat it all.