Jonathan,
Kingsbridge will be a good launch spot, we are going there on Sunday if you need a hand then.
I would say give yourself loads of time to get ready so you don’t have to rush as you miss a tide.
The trailer will need to be completely submerged before she floats off so you cannot attached a vehicle directly, a strong strop will work but you do not want this to snap! The jockey wheel takes a battering during all this, the load goes through it with no towing vehicle, so make sure it is really strong as well.
Ideally you have two helpers.
I have launched at Saltash twice with the trailer and it was fine both times.
Cheers,
George