Shouldn't be too difficult to get out of this - just put them in breach of contract. Find something they won't insure and ask them to insure you on it. They then are in breach of contract for not insuring you and you are entitled to full pro-rata refund.
This isn't internet myth, I got out of my insurance with direct line (i think it was them, maybe tesco). I found a minibus off ebay, got the reg number, rang for insurance telling them it was for a sideline as part of my business and they apologised and said they couldnt insure me. I then asked for full pro-rata refund and they obliged. I think I was around 6 months in. Can't remember what it did to my no-claims, was a few years ago.