'Goodwill' is an intangible asset that is built up over time by the owner of a business. In general terms it’s the value given to its good name and reputation.
https://www.politicshome.com/news/article/scottish-fish-traders-say-brexit-is-costing-them-over-1m-a-day-and-want-the-government-to-pay-them-back
https://www.politicshome.com/news/article/up-to-40-of-fish-sold-at-a-danish-auction-this-year-is-from-scottish-boats-avoiding-brexit-chaos-in-the-uk
In other words. When you leave an association, a trading or business relationship where mutual 'Goodwill' has been built up over years you can't leave that after annoying the other party with your demands for sovranty and fishing rights and expect the magic ingredient of a helping hand or smooth relations at the border to continue.
Join the real world. First fish. Soon car manufacturing. Inevitably financial services except the very dirtiest tax avoidance and share bet spreading schemes epitomised by certain firms with Tory interests and connections in the City of London. The post Civid world will be the Little Britain Going Nowhere Without Goodwill world.
Enjoy you "has been" populists.