If you booked through Disney UK the Illuminations viewing area is by the Morocco pavillion. The view there is OK, but you can't see the fountains very well and the sound is kind of behind you. The Wishes viewing area is on a lower terrace at the bottom of Main Street (can't remember the name), but the view from there is not good as there are trees blocking the castle. We choose to not even use our Wishes viewing area ticket.
I have also viewed Illuminations from the viewing area at the entrance to the world showcase (in between the 2 shops), this was the best view ever, the fireworks seemed to be coming overhead and you got a great view of the fountain effects.