there will end up being some sort of union of the various European States and it will include the UK.
What it will be, in form or substance we do not know. It will not be what exists today. There might still be a € Euro. the rules for crossing borders for bonafide Euro citizens and commerce will remain in some fashion, but different. The demise of NATO will necessitate a standing military of some sort to defend and to when necessary aggres.
The American Articles of Confederation were a failed attempt at amalgamation. the current system should be viewed not a an end but a reason for serious adjustment
The elites know exactly what form they want it to take. They planned the “two-speed/multi-speed Europe” decades ago, because they knew the first form would create a crisis. And any military that comes out of this already has plans for conflicts of aggression.