Since when were people rational? The PR effects alone are reason to break ties with the UK, similar to the situation with Panama after the Panama Papers. I'm already asking international vendors to setup corporate entities in an EU country instead to keep business as usual. When discussing this only from an EU negotiation perspective I think you're missing the psychological impact Brexit has had on us Europeans (Yes I understand many Brits still consider UK a part of Europe but so does Russia so...). If they want to leave, let them leave. It's over. Good luck, have fun.