Thorpe park is fairly flat, there is quite a steep(ish) hill going through the Colossus area but it can be avoided by going past the Crust Building and heading past Saw Alive (which is closed) - it will bring you out at the Saw area, or vice versa.
Don't think there are any other hills at the park really - although I don't think there is any form of disabled/avoiding the steps entrance to Rumba Rapids.