The answer to QDM's question is that the law in this area is based solely on custom derived from ancient usage. The right of navigation is different from any right of access enjoyed by a section of the public. The right of navigation extends only to ships. Any customary right of access for fishermen or a section of the public, for example to dry nets or take cockles, will be a right of access on foot only, not in vehicles. There could be no customary right to drive a dune buggy, or land an aircraft, on a beach. So I probably overstated the position in saying that it was arguable that there could be a right to land, other than in an emergency.
The best legal advice is that you should never place yourself in a position where you need your legal advice to be right.
