It's useful to use a broker for this sort of thing. They know where the best quotes are, and it doesn't cost any more than doing it yourself.
What you can do to help things along is to ask a surveyor to look at it. Whether you can find one on Skye I'm afraid I don't know.
Often, subsidence is purely "historic". That just means it happened a long time ago, and quite possibly stopped there. That would reassure an insurance company.
You could ask the current owner who they insure it with, and ask that company to give you a quote. Assuming the current owner has subsidence cover of course!