Fairlight Cove near Hastings has hard sand for the two or three hours around low tide, pebbles at higher tides. It's a fifteen minute walk from the car park. The beach is three quarters of a mile long and given its location has little or no passing traffic. Notices warn that access is at your own risk as erosion destroyed the wooden steps to the beach a few years ago. Recently new unofficial steps have been cut and access is OK with care. Usually twenty or so naturists on the beach and the occasional "textile". This is a genuine, get-away-from-it-all naturist beach, relaxed, friendly. First time naturist couples and single females would feel comfortable here. I have photos of the beach and the route to it.
Studland Bay in Dorset has acres of soft sand and hundreds if not thousands of naturists on a good day. Signs say it is an official naturist beach. Innocent naturists stick to the beautiful beach. Those in search of adventure venture head into the sand dunes.
There are many other naturist beaches in Sussex, Hampshire and Dorset (Google for "Naturist Beaches South Coast") but Fairlight and Studland are the best.