I think they are possibly wearing the caps as a joke. Apparently, in the UK fire brigade, white covers for peaked caps were primarily worn by senior officers or station officers to denote rank c1930s-1960s.
Maybe someone can tell if the men were senior officers by the rest of their uniform. I suspect not, hence why I suggest they are 'masquerading' as senior officers in the photo. And if the caps were someone else's and 'borrowed', they might not have fitted the individuals, making them look odd.
Big John - why the title "Just turned out of the canteen..." Is that part of the photo inscription (and possibly humorous) ? Or are they your words?