If this is your first time to Dubai, you only have 7 hours and you need to eat, then Dubai mall is the obvious choice.
You’ll get to experience one of Dubai’s absurdly large malls (which has loads of dining options for any budget), see the Burj Khalifa up close, see the fountains, the waterfalls and the aquarium too.
Taxi’s are super cheap compared with UK prices, and there is also the Metro which has stops at the airport and Dubai Mall. You’ll probably need to get a taxi back though.
You won’t have any trouble getting in to Dubai as a UK citizen.