Remember, you can't threaten the EU with loss of access to London Financial Services, because the EU gets to decide the exact pace and timing of any regulatory lockout.
Well, let's say the EU consult businesses and learn blocking EU data flows to the UK would hurt them.
EU's next question will be, "Ok, how long would you need to switch over to EU27 providers?"
Firms: "6 months."
EU: "Cool, regulation kicks in after 7."
There are costs involved in the above for everyone, and I'm sure EU businesses would prefer things stayed the way they are.
However, UK commentators picturing an EU in flames because it abruptly disconnects from London are dreaming.