depends what kind of beach you like
Woolacombe sands (nearly 2 miles long of flat sand) Croyde bay and Saunton near by)
North Devon
Watergate bay near newquay (good surf but watch the current) 2 miles of sand ), Newquay I dislike but the surrounding area has many good beaches and loads of camping sites
North Cornwall
Perranporth (miles of sand below a haven site) fairly good surfing)
North Cornwall
Bournemouth (3 miles of sand plus amusements/pier etc
Dorset
Personally I prefer some of the lesser known coves with smaller but beautiful beaches try for example
Trebarwith Strand near Boscastle North Cornwall
I am a real beach bum surfer type so know loads of beaches both family safe and a bit out of the way but very pretty (can have dodgy rips though)
happy holidays
