To think my partner should pay for the Christmas food/drink shop?

[santaclauseiscomingtotownnnn]

We both work full time and live together.
Have done for 3 years but aren't married and have our own bank accounts.
We said we would do the food shop and half the cost each.
The thing is after we said this I have been buying things and keeping them in freezer
Turkey,gammon,2 cheese bakes,Yule log,2 other nice desserts from M&S,M&S party food (8 packs so not cheap ) a bottle of gin,a bottle of whiskey,Malibu -a cheeseboard ,crackers ,3 tins of chocolates ,other chocolates ,crisps ,dips /pigs n blankets /ready made parsnips
This morning he said il see you after work
At least we are having the food shop so it's not as bad
There's hardly anything to buy !
Veg /fresh packs of stuff for sandwiches etc /maybe 1 more fresh dessert
A few more alcohol bottles

Aibu to think I shouldn't have to pay half when I've already spent nearly £200 ?

"Assumed" is the operative word here. You assumed, he assumed... Neither of you talked it through and now you are seething.
Use your words, OP.
"I bought £200 worth of stuff. I know we agreed to split the shopping bill and I am happy to do so but we will need to tot up the entire festive spend. I am happy for you not to pay for the food I already purchased but could you please cover today's shop".

Why did you pick up bits and pieces each week? I assume to spread the cost abit and not have a big shopping bill a few days before Christmas? That’s fair enough but it seems like you haven’t given a chance for your partner to do this too. Why didn’t you tell him to pay for some bits here and there like you were doing? Now he’s potentially faced with a big bill right before Christmas that maybe he can’t afford??

If it’s not to do with money though and there’s not much more to buy then why wouldn’t he just offer pay for this if you’ve already paid £200?

Have you even asked him?? Communication seems really lacking here.

I'd recommend using the "speaking to him" method of communication rather than the "assuming" method.

"DP, As I've already bought the Turkey,gammon,2 cheese bakes,Yule log,2 other nice desserts from M&S, 8 packs of M&S party food, a bottle of gin,a bottle of whiskey, Malibu, a cheeseboard ,crackers ,3 tins of chocolates ,other chocolates ,crisps ,dips, pigs n blankets and ready made parsnips, there's hardly anything left to get so why don't you just pay for the food shop later and we'll call it even, rather than us worrying about halving what we've spent on food?"

It sounds like you’ve just gone on with it without discussing it. You should have communicated in advance and made a plan.

She did tell him.
Why not read the OP’s comments before making assumptions

I don’t get why the spend wasn’t halved at the time. You go to Morrisons and do a shop for a turkey then go home and add the receipts to the list of things to be split. Maybe he assumes you have included it money he has transferred already.

I assume you don’t go to Morrisons together all the time and when you do they don’t split the bill at the till. So you either pay things separately and transfer amounts or pay alternatively for each shop.

Either way you created a SNAFU by making decisions without discussing it with him or getting an agreement. He may well think he has already contributed to these costs and you will have to show you bought them separately. He may think this is something you wanted to pay for yourself.

It's also a good idea to discuss a budget for these things. He might have been happy with a lidl shop rather than m&s

‘This way of doing finances’ works absolutely fine for us. But (unlike the OP, apparently) we’re capable of having conversations about this stuff. I’d just have said “Given that I’ve already bought all the meat, snacks, puddings and stuff, could you get tonight’s shop?” and DP would say “Oh yeah, OK then” and that would be the entire discussion we’d need about it.

It reads like she told him after making purchases - that's different to discussing a plan and making joint choices.