I am a therapeutic riding instructor in Canada, who is hoping to become a PT and do hippotherapy in the future. I'm not sure on the regulations in the UK, but in Canada or the US you have to be certified through AHA in addition to CanTRA or NARHA. All of these accreditations involve programs that you must work through (you can't just do the test). You may also require your BHSAI - in Canada certain levels of therapeutic riding instructor require the Canadian teaching equivalent.
Google the American Hippotherapy Association - they have lots of helpful information.
I would recommend that you seek out your appropriate therapeutic riding association in the UK and look at becoming a therapeutic riding instructor before going into hippotherapy. I'm sure that the UK has some sort of accredited program for becoming a therapeutic instructor. The information you receive through that sort of program will be extremely valuable, as well as exposing you to the field you want to work in.
I hope this helps!