I'm heading across from Exeter lads, leaving around half 7. Let me know if paths will cross although I have no idea of the route so will just be following TomTom...
Likely hood is that you'll take a coastal route - Exeter, Honiton, Axminster, Lyme Regis, Dorchester, Bere Regis, Wimborne, then cross country to Verwood.
Shaftesbury is quite a bit further north.
Although you could go A30, A303 upto Wincanton, then Gillingham, Shaftesbury?? Google seems to think it's 2 miles further, and will take you an extra 9 minutes.