I was surprised to find that general travel policies mostly cover riding. Coverwise were only £13 for annual European multitrip which covers loads of sports including riding / horse trekking, just not polo, racing or hunting.
I used Post Office Travel Insurance for my riding holiday in Crete. They were easy to speak to when I had to move the hol a couple of times due to the pandemic. I like that you get straight through to a human who can immediately give you the info and take the action you need, and no waiting for hours on the phone.