Strictly speaking, I think you will find that anything below the line of mean high water spring tides is owned by the crown.
I have landed several times in helicopters (as a passenger) on beaches above the high water mark while waiting for yachts to conduct aerial photography. As I remember it, the pilot always 'checked the oil' or something similar. We never heard a peep from anyone about it.
The 500 foot rule does of course apply. The CAA have prosecuted in several instances.....