Trouble is, things like chilli and stew and curry and jacket potatoes need cutlery and possibly a seat at a table.
I'd be tempted to get the bbq going. Add some warmth too.
Tons of hot dogs and burgers. Meat and veggies easily catered for.
You could jazz them up and do loaded hot dogs (so a pot of chilli that people can add a spoonful of to their hotdog. Or whatever else).
Then it's Just sauces, cheese, salads etc.
For dessert, I'd just buy cheesecakes, chocolate cakes, Victoria sponge. Pre cut them into slices. Jug of cream.
I'd be tempted to do a hot chocolate station too. Mini marshmallows, spray cream, sprinkles. All next to booze that you could add a slug of.
Having done a couple of outdoor parties in the winter, I'd also recommend some hay bakes. Can be used for seating but also, if the weather means mud underfoot you can spread it around to help with the mud/not slipping about.
Any room for a decent fire pit, or patio heaters?