I've ridden at West Bay before as well... def. recommended for Dorset.
I haven't ridden in Dorchester before but it looks like it could be good. I remember seeing someone riding a Fourplay at the BMX/skate park (which is decent if that's what you like)... there are always riders hanging about there. The park is right next to the station.
I know it's not as far South but central Bristol has lots to offer.
There's a popular spot near the Imax cinema and explore@bristol which I've been meaning to visit for a while.
Also, on the opposite side of the river to the SS Great Britain there are some really nice combos of walls, rails, gaps, pontoons and steps that have potential if you follow the river up towards the Oceana club etc.
I'm at uni in Manchester at the moment but come home to Somerset (roughly between Yeovil and Taunton). Any good spots in Somerset?