Anything with electrolytes - sports drinks, Gatorade (if you can get it, I didn't have to explore that option while in England), even Pedialyte...and it should be at room temp, to avoid shocking the stomach. Nothing like taking a huge chug from something that should help you and having your already abused stomach decide it's not going to play ball anymore!
You get hungover because alcohol dehydrates you. (You know, once you "break the seal", so to speak, you're in the loo just about once for every drink you have?) You lose a lot of things your body needs, like salt, potassium, calcium, etc....so any sport drink should set you right after about an hour or so. Don't forget to allow it time to get back into your blood once you've drank it; nothing is instant!
A handful of ibuprofen or paracetemol should help also, for the painful brain-banging that comes with the really great hangovers.
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(Maybe not a handful, but you get my drift...) Just don't take them *while* you're drinking....when mixed with alcohol, pain relievers can cause liver damage up to and including liver failure. Talk about a rough night out!
(On a side note, drinking quality booze helps as well. The vodka in a plastic jug with a handle and a poorly glued on label is a DEFINITE no-no!)