"The Crown Estate owns around 55% of the foreshore (between mean high and mean low water) and approximately half of the beds of estuarial areas and tidal rivers in the United Kingdom."
If so I assume there is an agreement in place to allow you to take wildfowl possibly through a club??
If so the club will have clear rules as to what quarry is legal.
If not through a club but with agreement from the landowner or his agent then you may be OK.
If it was me I would have a word with BASC to get their view.