I bet PIL is very grateful to you for doing this. I did it last year and it worked really well.
First, I menu planned for each meal we would be there (a whole week).
Then I did the online shop (Asda, since that is the only place that delivers to PILs house, which is almost rural). Remember to include Christmassy treats - my MIL loves chocolate ginger, for instance - plus whatever your DC will need (extra milk? Nappies? Snacks to tide them over if you're going to be eating later than usual for them? Ice?) and also think about what you'd normally pack - could you get it delivered instead, thus freeing up car space?
I booked the slot for 23rd, so I'd have time to pick up any crucial things on 24th that they couldn't supply. I carried on editing the order for ages after it was booked - no problem as long as you re-enter your card details. In the end they delivered on time with no substitutions or missing items that I remember. I was deeply relieved about this, because they were delivering all the alcohol as well as the food, and I'd chosen the drinks very carefully (Ocado has a much better choice of wine). The total bill was #300, but that was for a whole week (excluding meat, see below) for 4 adults plus some meals with guests, and DD. Given that there were 26 bottles of wine in there, I think that is very reasonable.
I asked PILs to order all the meat separately (beef for Christmas dinner, plus bacon, sausages, a couple of chickens) from the butcher for collection on 24th. This was the only bit that didn't work out quite as I wanted. I had expected the butcher to make the order up in advance, since other butchers I know do this at very busy periods. This one didn't and was putting orders together on the fly with a huge queue. DH and FIL hiked down there on foot because we were almost snowed in - they told the butcher how many we were and he gave us the correct amount of meat for that number - so no leftovers! I was really disappointed. To avoid this, call the butcher yourself and if necessary plan to collect yourself too. I don't remember the butcher's bill - most likely between 50-70 quid.
Having menu planned, I worked out what equipment I was going to need, and then thought through what I'd have to bring. These included the Magimix (heavy and bulky but worth it) and some serving dishes as PILs don't entertain. I also brought a few small food items from where I live (France) that I knew would be difficult to get there.
The whole thing worked really well. I really care about food, drink and Christmas so I was happy that everything I wanted was there, and I had the time to take over it because PILs were playing with DD, so they were all delighted too (MIL hates cooking so was relieved to have the whole ordeal - not to mention expense - taken out of her hands). DH got to relax, and we had a white Christmas. Perfect!