I don't know [member=198710]01smartc[/member] , but your post got me to thinking!
I can imagine that planners could be a bit cautious (even dubious) as regards a horse-sales business, especially if you do not have a stud farm, given the preponderance of folk who fill their fields with horses for what are essentially leisure related purposes!
If you are not already registered with HMRC as having a horse-sales business, then you should register I would suggest!
As to demonstrating an agricultural use, my head says that one type of animal would better suggest an earnest business endeavor (along-side the horse sales biz) as opposed to a hobby farm activity.
I would also suggest you cross all digits very tightly as regards gaining PP idc for permanent domestic occupation. That said, I wish you very good luck.