Why would Britian join another Common Market, if they just left one. This idea is a little foolish.
I don't see how you would construe their leave from the EU as opposition to being part of all foreign markets. We have a long-standing relationship with Britain and I doubt they would reject such a plan.
Indeed, Britain left the EU due to unacceptable political pressure applied by the EU establishment to rob it of its national sovereignty and right to self-determination. My biggest concern is that by assigning the parts of the United Kingdom to the regions of Atlasia, we are opening the door to subjugating it in the same way that the EU sought to.