Because it is new with no risk/repair cost/driver data to base a price on, therefore they will default to a higher insurance group. Since I believe that you are Scotland based I would call in, in person to your local NFU office. Okay if that's not local phone the local NFU ( not the call centre number) Just because it is cheapest doesn't mean it's the best. You only find that out when you make a claim. I have to declare a conflict of interest
the farm, contents, farm public liability, animals, horses, cars et al insured with the NFU ( vast premium so should be invited to their Christmas lunch and shoots. One of my neighbours was also a director)