"It's called planning."
Planning!!! I unclog my nose in the general direction of your "planning". So 20th Century.
Luckily we have a thrusting, up-to-date government, freed from hideously-outdated Waterfall dogmas, making glorious progress towards an Agile Brexit.
The Minimum Viable Post-Brexit Britain they're developing will have the same geological structure as now, so don't worry. It'll still be an Isle, and still be Sceptred. The rest is just minor UX tweaks. Anyway, Brexiters still don't have a clue what they want, so why waste time planning for it?