Hey, I'd say no. I'm y11 but done some gcses early and once they took my waterbottle away just because there was a price sticker on the side of it and I wasn't allowed it back :/ I think they only allow water as they're afraid people will put performance boosting drugs in drinks (hence why clear waterbottles) etc. and exam boards are all bothered about conditions being fair for all candidates. However if you really can't do a few hours without you could probably use a clear energy drink so it looked like water (probably not monster tho) in your bottle, or just drink it in the breaks.
As a separate note after gcses you really need to deal with your sleeping issues so you can drink less energy drinks, kind of sounds like an addiction if you ask me - they're not great for you lol