Blandford DH thing is UK bike park near Shillingstone. As said, Purbecks is on doorstep and you've also got Wareham forest, Puddletown, New Forest and Canford Heath.
Big healthy scene down here too, lots of group rides – 50+ riders on summer nights! 15-20 riders winter night rides, plus weekend stuff.
Also spoiled for choice for bike shops. Ride cycleworks, Bikelab and Primera all stock top kit, and all 5 minutes drive from me :-).
We also were about the only place not to get snow this year so that should make a change to Wales.