I'm in the process of having our bathroom re-done and asked the plumber about a higher loo.
The existing seat raiser is a horrible plastic one which I can't unscrew to remove to clean the loo. I can't put the lid down either. He suggested raising the existing loo up on a plinth, but instead he's found a 500mm high (to seat) loo for about £375 plus VAT. Actually thinking about it, I didn't ought to pay VAT on it, or maybe they would argue its not specifically for a disbaled person? The other option he put to me was one of those loos that hang off the wall, similar to a basin? You can put those at whatever height you like.
Carol