I'm not sure "Poole" and "cheap" really goes together! (or "boat" and "cheap" for that matter...). Parkstone Bay marina used to offer storage on your own trailer, and I assume you could then self launch, or just tow round to Baiter. Storage at a sailing/yacht club (Parkstone YC, Poole YC, etc) is typically a bit cheaper, but then you have to be a member (with associated fees and possibly other requirements/duties). Ridge Wharf and the others up towards Wareham are a bit cheaper, I don't know how much Shell Bay Marine charge. Worth checking with Cobbs and the other yards on that site (Davis?), as they used to have plenty of boats on trailers self launching.