I do not usually believe much of the opinions that banks pontificate, but HSBC seems to have it about right. The Guardian reports:
The pound would fall 15%-20% against the dollar. The UK economy in 2017 would grow at only the half the rate currently expected. Inflation would be above 5% by the end of next year, creating a thumping policy headache for the Bank of England. This is HSBC’s “central case” for what would happen if the UK votes to leave the European Union. Too alarmist? No, it all sounds entirely plausible.
Note that HSBC was making short-term forecasts. The bank’s analysts were careful to state that, regardless of the outcome of the referendum, the UK would remain a flexible and strong economy that would “eventually achieve a strong economic performance in or out of the EU”. The short-term impact, however, could hardly fail to be severe.And the short term impact would last at least two years, the time it would take to negotiate the UK's way out of the EU.