I've only read pages 1&6 of this thread, but it sounds to me like schools in the UK who are still enforcing blazers - indeed any inappropriate uniform for the weather conditions - need to be dragged kicking and screaming into the sunlight, so to speak.
How about a mass walkout organised by the kids in each school via social media, where at a given moment in each school on a certain day, all the kids who are exercised about this issue, get up and walk out onto the school field (if they still have one) to protest? Parents could roll up with some placards etc to support them, bring the local press...
I'm not a protester at heart, so I'm sure others would have good ideas. Maybe the kids in each school could organise a debate with the senior mgmt team on the subject? Petition the Ministry of Education?
Seems like an issue affecting a lot of people still, and one which just seems utterly ridiculous to me in this day and age. Britain is known worldwide (or is it just the Antipodes that think of you like this?) for having a bit of a stuck-in-the-mud attitude to change - so why don't you prove others wrong and FIGHT for recognition of hot weather sartorial requirements, instead of just whingeing about it each summer? G'orn, I dare youse! The kids would be rapt if they won a small victory like this - the Battle of the Blazers, summer 2017 
And for goodness sake tell them to remember that any change that's achieved, can only be accepted provided they have it in writing that the school uniform policy has been amended accordingly. Otherwise you/they will just have to refight it all over again next year.