Britain is facing massive reductions in tax revenues if it goes for a “Hard Brexit”, i.e. leaving the single market. The referendum voted to leave the EU, but it does not give anyone a mandate for leaving the single market, indeed the Tory manifesto states:
We are clear about what we want from Europe. We say: yes to the Single Market. Yes to turbocharging free trade. Yes to working together where we are stronger together than alone. Yes to a family of nation states, all part of a European Union – but whose interests, crucially, are guaranteed whether inside the Euro or out. No to ‘ever closer union.’ No to a constant flow of power to Brussels. No to unnecessary interference.
The referendum question was only about membership in the EU. There is no mandate to leave the Single Market. Apart from in the mind of David Davis, and possibly the political strategising by Theresa May there is really no reason beyond ideas about why people chose to leave the EU: immigration being the obvious excuse, negotiation being the more subtle.
If we let Theresa May dictate our departure from the single market, then we are allowing one person to rule over us like an out of control monarch and since sovereignty is granted to Parliament via democratic vote, there is absolutely no reason democratic vote will not get rid of this threat to our rights.
If the unelected prime minister is going to behave like a dictator, then the United Kingdom will fall apart.