Hot food is subject to standard rate VAT anyway even if bought in a supermarket. Eating in vs. taking away only makes a VAT difference for some cold food- from VAT point of view eating in means you are buying catering services AND take away means you are buying food.
Most retailers/restaurants do not price these differently but they have a way of accounting for it differently for VAT purposes. So they charge £12 for cold food which can be either eaten in (VAT is £2) or taken away (VTA is nil). So to the retailer they make an extra £2 in taken away. No need for a separate invoice.
With regards to McDonalds, probably only the salads would come under the zero-rated if a customer takes away. McDonalds ask if you’re eat in or takeaway but don’t charge the price of the salads depending on the answer -the price is fixed.
Hope this helps.