I only have public liability insurance through the BHS. Is it vet fees insurance you want? I used to have that with PetPlan but cancelled it when they doubled the price despite me not having made a claim.
The rates for vets fees vary depending on where you live and what activities you do with your horse. Probably the easiest thing is to use a comparison search engine like the one on the H&H website.
There are some to watch out for though. If it seems a lot cheaper than the others there is probably a reason! It would be worth checking out any provider by searching the forum for threads first.
Shop around all the one's with a good reputation as which is most expensive seems to vary. I'm with NFU and despite often seeing people saying other good companies have been cheaper for them when we shopped around for my son's horse they were the cheapest by over £100. They all have slightly different criteria for pricing so it depends where you fit best. Avoid the very cheap ones though - there is a reason they are cheap. Also make sure you read the policy details, lots of people get caught out because they didn't really understand what they are covered for.