Be careful about rider only insurance. I have it through BHS. I got membership for that purpose and got it for OH too. I thought that if we were hacking on public roads we needed some insurance just as if we were driving a car.
But that doesnt mean that I actually need it nor could ever use it. It is insurance of last resort for 3rd party liability when riding a horse (usually one that doesnt belong to you). Last resort means that if you have personal liability insurance with your Home Insurance Policy, that policy is the one you have to claim on first. If one is riding a horse belonging to a riding school, their insurance covers the horses and probably clients too.
If you are sharing a horse, your cover will depend on it being a genuine share where you contribute to the care and upkeep of a horse - it is not valid if you hire a horse per hour.