I didn’t know it before but across the channel from France and its Mont St Michel is the British equivalent, St Michael’s Mount. It was also once a religious place but is now topped by a castle, still privately owned but now a National Trust property.
Here is a photo(of a poster) of what is looks like from above. It’s one of those places that can only be reached by boat once the tide comes in. You can approach it from a causeway when the tide is down.