Hi Simon,
I sail in Poole Harbour, keeping my boat on a mooring at Rockley Boat Yard. There are several places you can launch into the harbour, but your problem as always will be where to leave your car and trailer.
There is a slip at Rockley. It is quite expensive I think for a one off launch. You can park there and I have seen people leave a car a trailer overnight. I leave my car overnight every time I go, with no problem. The slip and car park are right at the back of a huge caravan park, and there is a night security patrol, so it is safer than most places. Mind you, I could leave the keys in my old Subaru and I don't think anyone would take it. I don't know if they would let you leave your car there for an extended period.
There is also a slip well up the Frome river towards Wareham where I think there may be quite a large field where you may be able to leave your car, but I have never used it.
Poole Harbour or Christchurch are ideal BayRaider waters. Loads of potential exploring and landing places.