Inbound investment will collapse as companies need access to the single market. The economy will shrink as a result, taxes collected will go down and public services will have to be slashed. The 7% current account deficit won't be offset by continuing capital inflow, so the currency will collapse and Brits won't be able to buy as many nice foreign products as before.
Scotland will try again to secede from the UK, and Wales is completely fucked without EU subsidies.
The path to fiscal sustainability (government spending vs. revenue) will have to be recalibrated with massive spending cuts on public services. Trigger for widespread social unrest.
In the EU itself, this will probably trigger EU oppositon in Spain, France and Italy etc and possibly lead to the break up of the whole edifice. Probably a decades worth of instability, reduced investment and ending up with the Russians invading eastern Europe again.