The 18k miles is based on a 35mile each way commute, 32 miles of which are motorway. I think accountants are classed quite highly as we're generally dull & unadventurous!
although it sounds picky, the 32 motorway miles wont matter, 70 miles for the majority of your week is still quite a bit, most people i speak to will do between 8-10k for a variety of jobs, but same as most my responices to insurance questions, insurance isnt logical its statistical so no-one will have a price reduction at every years renewal and there will always be someone driving a more exotic car and paying less than you but thats insurance, plus the insurance companies need to make a profit on top of covering their costs, which in insurance is the likely-hood of something happening unlike other industries where the costs dont rely on ifs and buts