You will need you BHS stages (1-4)
Your PTT, You BHSII, and BHSAI...I think that is right I am sure you can find info about each qualification and exam on the BHS website.
If you click the link then scroll down the page there is a kind of flow chart of the exams, if you click on a specific stage it will show you the sylabus so you know what is required. Im wanting to do the same and im currently only upto stage 2, as I work full time its going to take me years
I dont want to work as an instructor though I am just doing it for the knowledge.
To become an AI you need BHS complete stages 1-3 (riding and horse care), then you do your PTT exam. If you pass you are awarded your PTC (preliminary teaching certificate). You must also complete a portfolio of evidence with lesson plans, self evaluation etc to hand in on your PTT day. You will also need to attend a first aid course and child protection course. To take your stage 2 riding you must take your riding and road safety test first. All the info can be found on the BHS website - www.bhs.org.uk then click on exams. Good luck