Maple Leaf Quarter Horses - They're on the west coast, I think a little north of Glasgow. They do western riding lessons and trial riding, or at least they did when I went to view a QH for sale about 5 years ago!
I know of a woman who used to be a livery near me, Western riding was her thing. Her Name is Morag Higgins, I dont know her personally, so I couldn't say anymore than that. She has her own yard now but I dont know if she teaches western. Might be worth a try, she does a lot of natural horsemanship, I think!
Maybe google Ross Dhu Equestrian Center. She may at least be able to point you in te right direction.