Andrew, I live on the outskirts of Poole about 5 miles from Bere Regis. There are several routes for you to choose most as long winded as each other and the choice would depend on your time of travel. If it were me, and all other things being equal, I would stay on the M5 and leave at Junction 25 taking the A358 to join the A303 going East. Leave at A3088 and bypass Yeovil onto A37 direction Dorchester. Join the A35 and follow directions for Ringwood/Poole etc. When you get to the roundabout junction of the A35 / A31 take A31 and at next roundabout take exit right and follow the road up the hill by passing Bere Regis town centre and the site is at the top on the right.
Have a great time in a beautiful part of the country. Ray