Buy Lucozade once airside of security, you won't be able to take enough through and very unlikely to be able to buy it in Turkey. I've been to Barcelona in September for the few years so the heat hasn't been as strong as in summer but still warm enough for the need for a FRIO pouch.
I've always done cabin baggage with Easyjet and been at the departure gate early enough to avoid being asked to put my bag in the hold. I've got a Doctor's letter in my passport but haven't needed to resort to it yet regarding baggage going in hold.