Camelbak aren’t worth the money. It’s way over priced mediocre gear. If you are going to drop £170 on a daysack, there’s far superior stuff out there for that sort of cash.
I think it’s a bit out of order that you are being forced to shell out for a bit of kit for a Cadet Activity, just for “uniformity” rather than practicality.
Classic Berghaus munro in olive green, around £70, MTP version is pushing £100. There are cheaper knock off versions in MTP for cheaper.
You can pick up MTP versions of the old NI daysack. Probably from your local Surplus Store for mega cheap. Good storage with 2 side pouches and 2 top flap pouches. Not the best option though, certain brands are a bit naff.
Karrimor SF are banging. The predator daysacks are large capacity, technically could be considered a Bergan with their zip on side pouch capability and usually cheaper But some new are around £100.
Helikon are usually pretty good, I’ve never used their daysacks though. But if their other kit is to go by, it’s good stuff and fair priced too.
You could get an olive green daysack, or even use your civvie one but use a waterproof MTP cover over it, for about £12.
Stay away from Cadet Direct. They are rip off merchants. Keep an eye out on EBay, there’s usually some bargains.