When the UK leaves the EU it will no longer be in the Common Fisheries Policy. The UK Government has laid out in a White Paper how it will manage Fisheries – the White Paper does not cover the marketing of fish. The White Paper is covered fully here:  Seizing Opportunities: The UK Fisheries Bill
https://theorkneynews.scot/2018/11/03/how-to-prepare-if-the-uk-leaves-the-eu-with-no-deal-fishing/