>>> If that is the case then we don't need a trade deal as such
You seem to be ignoring the reason why many non-EU firms (such as Japanese car manufacturers) have built factories and distribution centres (employing many thousands of people) within the UK. The main (or probably the only) reason that they're here is because they can access the EU market without having to pay import duties in France, Germany, etc, thus enabling them to sell their cars (or whatever) at more competitive prices in those countries.
If the UK doesn't get a deal that enables the free movement of goods between the UK and the EU to continue, the vast majority of such firms will close their UK operations and move their facilities to France, Germany, etc. That would result in high unemployment in areas where those overseas companies are currently the main employers, with knock-on effects elsewhere as the companies which supplied goods and services to them also lay off workers.
Many analysts estimate that unemployment will rise to at least 5 million people (with some fearing that it might reach 8 million) if the UK can't continue to trade freely with the EU. That would result in higher taxes to pay for all the state benefits to those people, meaning that those still in work had less money to spend in the shops. That, in turn, would lead to an economic downturn, lasting for perhaps 30 years or more.