Hi Darren,
As with a lot of things drone related, it depends.
There is specific permission to fly on Crown Foreshore and Estuary areas (in the absence of any other restriction that might prevent you from flying). As a full GADC member you can find these aread marked on Drone Scene.
There is also a thread discussing a specific area of foreshore, but it covers a lot of the principles, here: The Crown Estates clarification! Foreshore & Estuary areas - #10 by callum