I did start a ride in Witchampton, the woods consisted of double track and would suspect privately owned. The airfield at Turrant Rushton is an experience, last used in WW2 and you can ride around the perimeter. There’s also a memorial. Badbury Rings is close by and Ackling Dyke, a Roman road, leads to it.
Edit: yes, there is a loop in Moors Park but it’s easy single track. Very expensive to park, local roads can be used instead.
Disused railway track from Blandford to Sturminster Newton, cafe at disused station of Shillingstone. Haven’t been to Blandford Forest but it certainly has gradient.