I use the slip at Hythe Marina most of the time. There is loads of parking and they have CCTV and security guards so the trailer and car are pretty safe. Parking for the car and trailer will cost you £10 for the day - use of the slip is free.
Boat Launch Site 108
The slip on Bath Road, Lymington is pretty good. Its quite steep but there’s a huge car park.
Boat Launch Site 102
Camber Docks in Portsmouth is also good. I have never launched there personally but watched other RIBnet members recover from it.
Boat Launch Site 26