Order your turkey well in advance!
Plus:
Mincemeat (for mince pies)
Frozen pastry if not making your own
German chocolate covered gingerbread cakey things - from Aldi or Lidl(yum)
Stollen (as above)
Gravy browning
Snacks - crisps, crackers, nuts etc
Wine, beer, sherry, soft drinks etc.
And all the regular, non-Christmassy stuff of course!
Also, sprouts on the stalk keep much better than supermarket ones, so you could buy them a week or so in advance. Farm shops and greengrocers sell them. Keep them outside with the bottom of the stalk in soil. This will stop them drying out and also the frost will sweeten up the flavour.