I don't know what you're trying to suggest by that. VAT rules on food, hot or cold, take away or eat in are complex in their own right.
The point is, there is no import duty on british food used in the UK, but there would be on American food brought into the UK. And Macdonalds use UK sourced food.
(Actually I am not suggesting that they only use UK-sourced food because of the duty advantage - there's a whole load of reasons why they do, and principally simply because it's more cost effective. It was you who brought up MacDonalds and I merely point out the flaw in your argument.)