Well, Burnham-on-Sea/Berrow/Brean in Somerset are on a beach that's around six miles long, Brean Down is lovely to walk on, with views across to Cardiff and North Devon. I love it there, used to go there as a kid, and walk for miles along the beach. You have to be careful of the tides, it has one of the highest tidal ranges in the world, around thirty feet, IIRC, and there's very dangerous mudflats if you venture too far out, like far too many grockles do and get stuck, or their cars do. Stay on the sand and it's great, there are kite boarders there, and a lot of the beach is empty of people. Beesands in South Devon near Kingsbridge is lovely, but very small and tricky to get to. There's b&b at the Cricket Inn in the village and in Beeson just up the hill, and Torcross and Slapton Sands are also pretty close.