No catch. They only insure special cars. They do not insure daily drivers. If your car is used every day, then Grundy is not for you. You can read up on their FAQ page.
They have unlimited milage, but that mileage is not back anf forth to work. They consider the travel to and from events, driving the car around on weekends for pleasure, and to and from shows as being the proper use of a special vehicle. They also expect the car to be kept in a locked garage when not being used.
They are very easy to work with. You should give them a call and get the specifics for your situation.