ISle of Purbeck in Dorset, lots of sandy beaches and lots of options.
Swanage - nice stretch of beaches, once launched head to far end of beach and you will find it deserted even in the height of summer. I often anchor down that way and swim assure.
Studland Beaches - Knowle Beach, Middle Beach, and the other One South Beach, same beach that goes from Poole Side all the way to Old Harry side, studland bay, pub 100 yards walk from south beach !
Poole Harbour Baiter - then you can go across to brownsea island
Lulworth cove, bit pebbly