Before I sat (or rather taught) for a BHS Preliminary Instructor teaching exam, I had to complete BHS stages 1 & 2 in both care & management and riding, in addition I had to have BHS stage 3 care & management, BHS Road Safety and a first aid qualification if I wanted to go on the register.
There are specific time limits between taking the exams to show that you have also gained practical experience. Have a look atwww.bhs.org.uk for more info.
Some agricultural colleges want you to have minimum BHS Stage 1 before they will accept you, so check out the entry requirements as well. Also it depends how far up the teaching route you want to go as you will need to pass more exams before going applying for further teaching qualifications.
Hope that helps.