I hesitate to continue to post on this locked thread but I have to correct Aidy's assumption. The EA has no legal standing in this issue and therefore their reply is not relevant legally.
The legal position is as follows for any land around the UK:
All land has an owner. Very broadly speaking, anything on that land belongs to the land owner unless specific rights have been granted to others.
Above high water mark the owner will probably either be an individual or the local authority.
Below high water mark, up to and including the sea bed, the most likely owner is the Crown Estate. Alternative owners may include the local authority or the Duchy of Cornwall.
You may not remove anything from land without permission of the land owner. Otherwise it is potentially theft or other more specific offences. Seaweed is included in this rule.
So, as Aunt Sally has said, first you must identify the owner of the land. Best to contact your local council and ask a) who is the land owner of the area above high water mark, and b) who is the land owner of the foreshore i.e. between high and low water marks. If they cannot establish themselves as legal owners, then you can assume it is Crown Estate.
Once you have establised ownership, then you can determine from the owner what permissions, if any, apply with regards to the abstraction or removal of seaweed from above and below the high water line. For land owned by the local authority, bye-laws will be the best evidence of general permissions.
The situation will differ around the UK.
Note: The Crown Estate have given some guidance here https://www.thecrownestate.co.uk/rural-and-coastal/coastal/seaweed-harvesting/Collection for personal use does not require a licence and we are content for such collection to proceed for small quantities. We recommend anyone doing so takes account of environmental sensitivity of collecting anything from the wild
