I insure with PetPlan - I was with NFU who were fabulous, but I wanted a cheaper insurance and Petplan offer different levels of vet fee cover and also varying excess. I opted for £3,500 in vet fees and £500 excess as I figured £4k was a reasonable level of cover especially when I have 3 insured with them.
Actually 2 now as I sadly lost my mare. They will insure up to 25 years old as long as you insure before 20th birthday too.