It massively changes asset prices already built into contracts (what you would call expectations.) These people will take the losses on the balance sheet but it will also affect the total supply and the total demand of the UK for sudden asset shocks like this will change the behavior of a whole lot of people in the UK and also outside the UK due to the need to absorb losses of those assets.

Let's put it this way, the IMF estimates a 5 to 8% GDP drop if the UK does brexit without a trade deal that would more or less let those assests be priced the same prior to Brexit. For comparison in the US real GDP contracted by 4.2% between Q4 2007 and Q2 2009 according to the Department of Labor with the Great Recession.

Now a brexit with a trade agreement is still expected to cost the UK economy 2.5 to 4% by the same IMF report but that is a much smaller thing.