Lots of options.
DDS - Dorset Diving Services in Poole is the local shop, Been running for years, very BSAC orientated. Pop in they will link you to a few clubs if that's what you want.
A handful of decent boats run from Poole which you can book on individually: Beowolf, Rocket, Sha-King.
Swanage is very active. Shop on the pier is Divers Down. they run 3 boats on a shuttle off the pier to local wrecks and reefs.
Ditto Weymouth and Portland, lots to choose from.