Trim their sails. Do this by first eliminating all income, and wealth based taxes, so no individual, corporate, or entity based income taxes, and not estate, gift or other wealth based taxes. Second, Britain to become a zero import tariff nation. Third, allow a more free movement of people into Britain after basic vetting, but retain the ability to sever ties to immigrants if they behave criminally. Limit immigrants access to public assistance for a set period of time.
Then let the EU know that Britain will not be attending any negotiations. Reasonable heads will want to be reciprocal to Britain, but impose some tariffs, the unreasonable will want to "hurt" Britain with high tariffs.
The reality is import tariffs only hurt the people of the country imposing the import tariffs. So, these fools will be hurting their own people to spite the Brits, making real the metaphor of cutting off ones nose to spite the face.
But fools cannot be reasoned with, bargained with, or convinced. Let them be the fools they are.
The British economy will do well under this arrangement, and many EU corporations will seek to relocate to Britain if only for the tax benefits. Plus, the EU is a dying trade block, and that will not be changing anytime soon. Best for the Brits to cut their losses, and move on to greener pastures.
I had barnacles once, they were ok, a bit rubbery, perhaps they are more to the taste of the Brits.