Depends on the value of ‘plunge’.
The BOE have done sterling work (pun intended) in keeping the UK market on a level keel, their report published yesterday is yet another example of that work.
The central bank are of the opinion that Financial Stability will remain through what they describe as a disorderly exit, but they have a caveat:
Financial stability is not the same as market stability. Significant volatility and asset price changes are to be expected in a disorderly Brexit.
In a disorderly Brexit, a range of UK asset prices – including the sterling exchange rate, equities, corporate and government debt and bank funding costs – would be expected to adjust sharply, tightening financial conditions for UK households and businesses.
(BOE report July 11th)
It is how UK business has positioned itself before any such volatility that is important - I would guess that most are now prepared often by reducing exposure via investment until the outlook becomes clearer.