Tintagel1, the mythical birthplace of King Arthur has everything you'd ever need to shoot a complete movie: sea, waterfall, ruined castle, caves, which disappears with the high tide, green land. It's magical, beautiful and lovely; if you have the chance, visit it after all the tourists are gone and you're alone at the place.
If you go late in the afternoon there is a low tide and you can enter the cave below - the cave that, according to the myth, was Merlin's place.
(Oh, by the way: this entry was written by Peter Molnar, and originally posted on petermolnar dot net.)