The thing is, putting the "fastest" option will probably send you on the major roads. However, there are a few things to consider.
First, there are not that many "Major" roads in this part of the country and if you do use them, then it will quite likely add a few extra miles onto your journey. The difference in these cases between "Fastest" and "Shortest" could be considerable.
And, dependant on the purpose of your journey, you will miss out on some of the beauty of a rural environment and the surprises along the way. Not often you come across small picturesque country villages and thatched houses in London. (Not many cows either)