Think this might have been done before, worth doing a search for it. Any way where do you want this campsite up in the highlands or down in Cornwall.
You could try Mercury down on the Hamble. Theres a slip at the Marina and the campsite is 100 yds away. Have a look at boatlaunch.co.uk. Theres loads on there. Not telling you where my one is. It gets busy enough as it is! Well I might if you get stuck.
I don't know of one with launching facilities, but I do know of some just outside of St Tropez which have launcing nearby and you can moor/ anchor 100m of the beach. Quite a few people do it and there are places close by to move to if the weather turns rough. The campsites are right on the beach. Last year we were 100m from the water across the sand.
We have always taken a dinghy down and launched over the beach, but this year we're taking down our 25' trailer sailer... Not over the beach though!!
Here are two sites I have visited, one in France the other in Spain www.campingdesmures.com www.campingalmata.com
I tow a similar sized boat to these places, something for everyone there, if you need anymore info just PM and let me know.
Regards Dave