Mark's and Spencer's is a UK chain of shops famous for womenswear. It also has a food section. It is NOT a restaurant. The "dine in for 2" is a promotional campaign where they promote eg two ready-made meals including a dessert and a bottle of wine. But this is for you to buy in the shop and take home and eat at home. So dine-in, which as other teachers have said, means "have dinner INSIDE the restaurant and NOT takeaway or delivery" here means; dine at home. The point of the campaign is to encourage people to eg 2 people to go on a dinner date, but not at an expensive restaurant, instead at home.

'Dine in' means that you'll be eating at/in the restaurant. 'Dine in for 2' would mean that two people are going to be eating/dining at the restaurant. The sentence is weird, but it's the last part: 'in Mark and Spencer's.' Usually we'd say 'dining in AT a restaurant/a place.' Also, I'm assuming 'Mark and Spencer's' is either a restaurant or it's at the house of Mark and Spencer. If it's at their house, then they are sort of making a joke, as we normally 'Dine in' at a restaurant rather than at a person's house.

to dine in = to have dinner at home and not at a restaurant. (American English).

If this is an ad for a restaurant, then to "dine in" means to eat inside the restaurant instead of getting "takeout / take-away". "For 2" means that this offer is for 2 people. I am guessing that Mark's and Spencer's is some type of restaurant. Good luck!

"dine in" = not "take out"