You are perfectly at liberty to take for example a bike you bought elsewhere to your LBS that happens to be a dealer in that brand and they can deal with it under warranty. Same goes for taking a car to your local dealer under warranty. Should be the same for a Camelbak if they are an authorised dealer.
Your rights apply to the retailer you bought it off however, so should be first port of call, but any official dealer can deal with the manufacturer’s warranty. It’s their discretion though. There’s no harm in going into Halfords saying you bought it elsewhere, but here’s the receipt and as you are a dealer, etc. Can’t see Halfords saying yes though. The monkeys have no authority so have to call the kid (sorry, manager) in the back who doesn’t want the responsibility or hassle of sorting it out.
Anyway, should point out that Osprey apparently have good warranty service. People have told me they’ve had replacement parts or entire bags no questions asked, no cost. Something I should get round to doing then as the nipple has split.