If somebody is changing from running a livery yard to a stud, do they have to change the use of property officially? What other official things would they have to do legally?
My feeling is "yes". What you would have to do is another matter. I would approach the council to "test the water". Objection probably would not be aproblem, but it is worth checking. May just be a paper exercise.
If you are not making any material changes to the buildings and the property has planning permission for commercial equestrian use (including permission from the highways department) then you should have no problems. What your surveyor needs to look at is the original planning granted to the property including permission from the highways authority.
For taxation purposes you should look at Butler and Co website they are experts in agricultural. There are rating concessions on small studs and also taxed income can be set against for up to ten years.