Re: Brexit means ...
"The Chief Exec of the London stock exchange suggested that if we lose Euro Clearing, then that's likely to go to New York."
Because NY is member of the EU Single Market, right? Euro clearing is wholesale and can happen wherever people want to do business with each other. There's nothing the EU can do about that unless it wants to de-certify NY, HK, Singapore, Dubai, Beijing, etc, etc. That'll really work well for euro liquidity.
Why don't we just face it that "City bosses" including the Chief Exec of the London stock exchange are putting out a negotiating position to the gov't from their particular bit of rent-seekery. They've had it good and now have to sing for it. No sympathy here, but passporting & clearing certainly aren't the issue here.