In reply to baron:
We may be able to operate under the WTO rules, but the cost of non-tariff barriers to trade are estimated to be 10%-20%, completely dwarfing any tariffs. And they don't cover financial services, which are a substantial contributor to our GDP. If (and it looks likely) we have to go down this route, expect a shrinking economy, job losses, lower tax takes, poorer services / higher taxation. I guess this will help with immigration as no one will want to come to the UK.