I don't do individual insurance just third party, and I found it more efficient to pay £50 a month into an ISA to build up a rainy day fund for vet bills than pay that per month for 10 years and then not get paid out because the pony was over 16 by then.. Plus having more than one pony, if I paid that per pony over the same time period, and got one major claim paid out for vet bills I'd still be out of pocket, and only insuring some would all but guarantee the sick one would be one of the others

This way I insure equivalent of one and cover any that need it.
I also wouldn't send one off to the Dick for an operation unless I was absolutely convinced it would have a good chance of surviving plus had the capacity to cope with the risk of increased trauma by loading and travelling it in that state. I'm not sure a colicking or GS suffering animal benefits from the journey and unfamiliar surroundings let alone the treatment and all that goes with it. I may be hard hearted nowadays but I've done both and tend to call it a day at home rather than add to or prolong the suffering on a maybe.
Anyway, that wasn't your question, but an ISA or Premium Bond might be as good as most insurance policies when it comes to vet bills and loss of use payouts, worth a thought.