I have just added this to the map of places to fly your drone at Drone Scene:
Land owner permission requirements unknown.
Not for the faint hearted, this is a long flight out. Launched from disused boats near Marina. Used my M2P and practically one full battery. There are two other forts in the Medway they are all accessible if you had a boat/canoe or similar. Other photos online show persons camping on them.
The originator declared that this location was not inside a Flight Restriction Zone at the time of being flown on 15/01/2020. It remains the responsibility of any pilot to check for any changes before flying at the same location.