Pagham Harbour (Chichester) and Langstone Harbour (Havant/Hayling) both have reasonable mud with easy access although they aren't especially "private". But it all depends what you are looking to do - a typical naked shoot probably won't go down too well but if it was more of a "trash the dress" thing that's a bit more mainstream people won't likely be that fussed about it.
Love you, too
