Ok I second Indiths belly pork suggestion. that is really cheap and really, really delicious. It would be a good thing to have instead of turkey as you could have a huge piece with tons of leftovers for a fiver.
Or buy a frozen bird, if you buy a bird you can get quite a few meals out of it with stock and curry ect. Potatoes, carrots and parsnips are cheap at the moment as they are in season.
Make smoked mackerel pate as an appetiser and serve it on melba toast or home made bread, that should cost about 2 quid for the two of you.
Or make a lovely soup and serve with home made bread. make lots and put half in the freezer for later.
Buy Asda Cava for about 3/4 quid it is very drinkable or lidl prosecco, you and dh deserve a little bit of fizz at Christmas.
Also Asda do 3 bottles of wine for a tenner and sometimes they are quite decent wines.
What is your favourite dessert? If you are not mad about it, just buy some nice chocolate and some nice ice cream rather than go to the expense and trouble of making traditional trifles and puddings and cake. HTH