"I think it is great when schools take a hard line on phone use.
Still, if I were in your situation, I would get him a £5 PAYG phone"
That type of comment makes me want to Howl at the hypocrisy. A phone going to school is always terrible except for all the times when it's extremely useful and absolutely essential, worth breaking all the rules after all - pah!!
It's not hypocritical at all. The poster said that schools should take a hard line on phone USE, not on phone taking-to-school.
I think a super basic dumb phone with a few calls in it, hidden at the bottom of the bag, satisfies the "intent" of the rule. It has zero distraction temptation, and is not going to result in a mugging either. I got one for 20 pounds when I was visiting the UK a little while back.
And yes, do get him phone cards as well. If a phone is barely ever used, there is a risk of forgetting to charge it up (I speak from experience!). And while phone boxes are hard to find, you can usually find one if you walk around for long enough. Cards could help out in an emergency when the dumb phone is not charged up.
Not sure how GPS watches work, but if they are flashy/distracting/desirable property then I would say that they would create some of the same potential issues as a smartphone. Dumb phone hidden in bag would be better.