Teen Book with Two Boys Transported to Future

Question

I am searching for a book I read as a young boy (1980's to early '90s). I don't really recall many key facts other than I think it was either one boy or two brothers, from possibly war time, being transported to the future where they attend a school. At lunch they scan their wrists to pay, and the time projects on their wrist. One of them had a taste for a dessert that, when you peeled the lid back, turned into a spoon to eat it with.

Answer 1

Per sq33G's earlier comment, the book you're describing is almost certainly "A Tale of Time City" by Diane Wynne Jones;

Pots that have lids that turn into spoons;

The pots, Vivian found, were sort of stuck to the air. She had to give a little pull to get them loose. She discovered that you peeled back the lid, and if you needed a spoon or a fork, the lid shrivelled itself into a spoon or fork shape. Algae soup was not at all pleasant, like salty pond water

Paying by scanning their wrists

“I’m paying,” said Jonathan, and recited a string of numbers. “Yes, but are you in credit?” said the waitress. “Show.” Jonathan pressed one of the buttons on his belt and held his hand out with a row of signs shining on his palm. The waitress looked, nodded, and pressed buttons on the pink matching belt round her pajamas

There's a complete (and I assume copyright friendly) version available online here.