Likewise, each insurance company will give you a different price for insuring the same vehicle.
The Association of British Insurers (ABI) insurance group ratings are purely advisory. They are used by many motor insurers as just one of the many input factors in calculating premiums. As such, they can provide a useful indicator of the likely relative cost of insuring a particular vehicle. Insurers are not required to follow the advisory group ratings.
http://www.abi.org.uk/carinsurance/default.asp