oh dear, whenever i think of Alton Towers I think of burger king, chippys and hot dogs :/
First option you could prepare something from home if you have time.
Have a look at pps for burger king if you have the eat out book and maybe opt for a fruit bag instead of fries.
There are loads of different places to eat in the theme park though I've only been to a couple so have a look into it it's probably on their website, it was the last time i looked.
Or you could wait until the night and eat in the hotel there are probably some healthier options in there...
Have fun anyways, i'm well jealous lol xx