The Crown Estate foreshore (defined as all of the land between mean high water and mean low water) does allow drones to be flown. So you can fly there. Metal detecting & drone flying | The Crown Estate
This thread should also be helpful:Takeoff, landing & operating permissions, what is the letter of the law? To summarise it, the laws which are concerned with the permission of landowners is trespass law. Basically, if you enter somewhere without permission, and you are asked to leave and you don’t, you are trespassing. This is a civil matter, not criminal.
So one might argue that, so long as you leave if and when you are asked to do so (and you’re not breaking any other laws, e.g. bylaws or PSPOs or the drone regulations), then you can fly wherever it is safe to do so.
[Caveat; I am not a lawyer, and therefore I am not qualified to give you legal advice. Please do not take anything I say as as such]