I'm just relaying what I was told by the insurance companies. They say it's illegal to own a plated uninsured bike even if parked in my garage. They almost went as far as not wanting to insure me because they say it was an illegal act. Sounds as stupid to me as it does to you, but apparently it's the law.
Think about it. The MOT was told that vehicle was isured with xyz insurance company. They issued plates based on that info. You agreed to keep insurance on the vehicle when you got the plates. Now it is not insured. The plates are not allowed to be on an uninsured vehicle, right? Does that not make it illegal? You are supposed to return the plates if you cancel your insurance.