From the Crown Estate Website
Anybody wishing to carry out metal detecting on our foreshore requires a permit. This permit does not apply to the seabed or river beds or any other Crown Estate land.
We generally seek to encourage access over, and responsible use of, the foreshore and therefore make no charge for the permit. Foreshore in England, Wales and Northern Ireland is defined as the land between mean high water and mean low water. In Scotland the definition is between mean high water of spring tides and mean low water of spring tides.
Why is consent required?
All UK foreshore has an owner. Unlike the public rights of fishing and navigation and our general permissive consent for public access along the foreshore, other activities, including metal detecting, need the landowner's permission.
We are required by the Crown Estate Act 1961 to manage our foreshore and regulate such activities and this covers the locating and removal of finds, including those that might be identified as 'treasure'. The Treasure Act 1996 deals with the definition of treasure, along with the payment of rewards in relation to permitted metal detecting. Items which are not defined as treasure and found in the course of metal detecting are in principle also owned by the landowner.
Full Story and Application Form