Part baked bread rolls, pickles and salad (inc coleslaw/pot salad) to go with the ham after Christmas.
Cheese board and cracker selection
Fruit - grapes, satsumas, figs etc
Nice soft drinks - appletiser, schloer type thing
Goose fat for the roast pots and yorkshires
Pigs in blankets (unless thats on the butchers list)
For the gravy if you can make it then do, if not what I do when I'm doing a 'mid week' roast is buy the cartons of good quality ready made gravy, but add it to the roasting pan to take up the juices. (If you buy the store cupboard gravy rather than the chilled, then you don't have to use it and can have a bash at doing your own, but its there should it go wrong)
Might be worth buying some cartons of fresh soup, pack of bacon and sausages and bread buns to stick in the freezer, so that if you end up with family poping over or staying longer than planned, you can throw together some bacon sandwiches, or soup and rolls for lunch.
I think as long as you have your Christmas day food, you can pop out for food, most supermarkets will be open from boxing day 