Back up the thread a bit but we do a fair bit of risk management and mitigation for clients (not moving businesses but risk is risk just like Brexit is Brexit)
Making a potential structural change to a business process due to a potential change is usually based upon hedging of bets v cost
So first question will be “if this change occurs what is the impact” and in this case it is potentially significant to certain banks. Second question what is the cost of mitigating that risk both in people and finances and in this case the banks can afford to relocate and take people with them. The last question is likley do want want to leave any resources in the high risk place to retain some foothold if the change does not occur?
Looking at it from this perspective with my risk “knowledge” I think all banks that may have a issue will probably take a 60/40 split. They will move 60% + of activity into the EU and leave a footprint behind- if Brexit is grim they will slowly move the remaining business to the EU if it’s good they will leave the footprint in place.