It’s happening in my area, too (US, not UK).
I was also surprised it hadn’t been the case. Apparently there were some policies against phone use during class but the enforcement was so toothless and sporadic that teachers and kids alike were ignoring the rule.
Now the rules are firm, universally applied, and have actual consequences. That last part seems to be the key. You can try to say phones are banned but until there are actual consequences it’s not really going to make a difference.
Round here they have a locked pouch they have to put their phone in during school which seems to work reasonably well (although I'm sure not perfectly). It makes it more clearcut if they do find somebody with a phone not in their pouch anyway that they've definitely broken the rules. They get locked at registration at the start of the day and then unlocked when they leave school at special points.