I would've been around 10, so late 1970s. It was a bit post nuclear or similar war. We weren't actually told. It was a bit futuristic. Children being transported from all they knew, taken abroad, it was hot. The title was something like a couple of letters followed by a couple of numbers.
I know that is very vague, but it was one of the first books I read that wasn't Enid Blyton's or horse books or Watership Down or what all the other girls were reading. And I loved it.