Yes we are allowed to drive somewhere for exercise, what we are being encouraged not to do is drive a long distance to a beauty spot or other place of congregation, where, as Carooline says you then end up with a lot of people in the same place and the virus spreads.
Driving 3 miles to the sea, providing it is somewhere quiet, is entirely permissable.
The problem with the current situation is that there will always be a few officious jobsworths who feel that whatever the regulations they are proving their worth by making them more strict than they actually are, like the Trading Standards Officer who told a corner shop owner that he was not allowed to sell Easter Eggs because he was only allowed to sell essential foods. At least that one was promptly squashed.