The Micronation of Sealand is only the tower at Roughs off Harwich, which, lying outside of British Territorial Waters can be technically declared as independant - and has.
Shivering Sands, as reported on the Wiki, lies further south at the Thames entrance. At various times it has been occupied by "pirate radio" stations and at one point, it's said, there was even a gun fight over possession there. As far as I know this all ended decades ago when the UK formally asserted that Shivering Sands and Red Sands etc were inside territorial waters and put a stop to all the malarky.
I've sailed past the Sands and it's an awesome sight even at a distance. There's something surreal about its shape and isolation in what looks like deep sea.
It's one of the things I really like about the UK. Just about everywhere you go there's some sort of historical wartime "something". Not an impressive "something" always but...