[bertil]

> So what mechanisms keep the price from instantly dropping if "everyone" agrees it should be lower? Market inefficiencies? Or lots of money betting on an upswing?

Asymmetric uncertainty: some people believe there is a small chance that things could change for the better; in that case, the market will go up spectacularly and Apple will update their prices again. If that doesn’t happen, Apple doesn’t have to change their price again, which is, I presume, a fairly costly operation — but the equilibrium rate with an effective Brexit is lower than where it is now.

Once such situation is if the Parliament refuses to approve the referendum: Courts have rules their vote is needed, and most MPs were against Brexit beforehand.