What's the tidal access window on the slip at Levington?
I have launched both a 7.5metre yacht (with raised keel draught 0.7m) and my rib at all states of tide without any problems, using a car.
They have a steel cable and pulley at the slip if you dont want to use a car on the ramp. Attach one end of cable to trailer and the other to towbar etc.etc.
It helps at low water to have either someone to signal between car driver and person on rib, or walkie talkie between the two, or mobile phones, as we use on hands free ear pieces.
Hope this helps
PS That weekend is the Felixstowe regatta based from Levington (Suffolk Yacht Harbour) first race will start from 10.00 so boats will be leaving marina from 09.00, with usually 50-60 yachts competing so it will be very busy, although few if any will be launching via the slip.