Find a company that insures projects & SORNed bikes. As it avoids all the risks associated with being used on the road I would expect it to be significantly cheaper.
Try Bemoto or Bennetts - one or the other offered me project insurance when I was last shopping around for quotes.

Or SORN it and don't bother insuring it.

Or see if you can get it insured with your household contents as a collectible antique?