To become and AI you will need to complete your BHS Stages 1-3 complete and also your PTT, you then have a portfolio to complete, once this is sent off and approved then you get your full BHSAI. If then want to carry on you can take the intermediate teaching exam and gain your BHSIT, to become a full BHSII you will also need to pass the stage 4 complete.
Once you have this then you can do your BHSI (which I have just done!)
The other option is to go down the UKCC route or the ABRS route.
However, the BHS qualifications are more reconised.
You can do it through a BHS training school and exam centre, however, if you are under 19 you can go to college and do the diplomas in horse care and riding for free. ie. college course.
The college course diplomas will enable you to do the PTT and onward through the BHS, saving you many thousand of pounds, but go to one which is IV'ed through the BHS and NOT through the city & guilds as the training can be variable.
No you don't have to go to college. You could be a working pupil at a yard/ RS or pay to put yourself through the exams with private lessons at a where to train centre and self teach the stable management (the later is what I did)
Lots of places that I know of preferred working pupil over college trained. I trained at hartpury but was lucky I knew the YO of local rs/ livery yards they employed me as they knew me but did say of they did not know me they would have gone for the applicants who trained via working pupil route.
Am not saying all will have this view that's just my experience although I did learn a lot at college
You can also go the equestrian sports coach route.
I did my stages 1-3, i got my grooms diploma, i got really disheartened with the BHS though.
I then did a preliminary teacher course and did my level 2 sports coach training.
You need to do a portfolio of 4 linked lesson plans for your students you teach.
These plans need to be assessed by your mentor and signed off as up to standard.
You will then be observed teaching a lesson by an external assessor before they decide whether youve passed or not.
My stages got me direct access to the level 2