As hardline Brexiteers stepped aboard a boat on the Thames to throw haddock into the river in protest, I watched in total disbelief at the infantile extremes we have reached in politics. But actually, this week’s Brexit soap opera has finally brought the discussion to a crucial point: what exactly is trade?

People in the fishing industry are not happy because the Common Fisheries Policy to which Ted Heath’s Conservative government agreed in 1974 has, in their view, gravely damaged their industry. They want to be out of it, so that they can fish more freely. This was one of the many promises the Leave campaign made during the referendum, and like the £350m for the NHS, it turns out it isn’t going to happen.

If you ask people in fishing what they want, it is to leave the CFP. Crucially, it is also that they don’t want any new tariffs or taxes or barriers to trade, and actually they will need lots of EU migrants to come and work with them in order to complete all this extra fishing they want to do. Basically, they want the single market membership we currently have.

And so we reach the crunch. In fishing, people want to fish more (and by necessity stop other countries fishing so much, as there is only so much fish) with no new costs or barriers to what they can sell. But we share waters with the EU and Norway, and sell much of our produce to them, so we are going to have to reach some agreement about who fishes how much and how we sell it to each other.

Here the Brexit ultras reach their fundamental misunderstanding – or deliberate ignorance – of trade. They seem to think that outside the single market and customs union we can put up huge barriers to imports but expect to be able to export without any new barriers. The Leave campaign told the fishing industry that we could leave the CFP, yet failed to explain that they also wanted to leave the single market and customs union. Fishing businesses would have to cope with new tariffs, taxes, customs checks, and lack of European workers’ labour would follow that.

To be honest, it’s pretty simple. If you put up walls against imports from elsewhere, don’t be surprised if it is harder to get your exports out.

This exact drama will eventually play out in every British industry. Leaving the single market and customs union will mean new rules, new taxes, new tariffs. Every industry will need to adjust, whether because the tomatoes they used to import from Spain now cost more and their margins are squeezed, or because the fish they want to sell to the mainland is unattractively pricey. And new trade deals with Japan or the USA will not cut it, because we do almost half of all our trade with the EU.

We don’t have to do it like this. If we stayed in the single market and the customs union, jobs would be safe and businesses would have the stability they need to plan and invest in the future.

For my city of Liverpool, as a port town with a long history of trade, we understand this well. It has already been particularly stark for us at the Vauxhall car factory, where hundreds of jobs have been lost. You only have to look at the number of times the different parts of a car move across borders within the EU as they are being made to realise what simple customs checks (let alone trade tariffs!) would do to the process. It seems obvious that businesses can’t afford it. Why make parts in Liverpool when you could make them in Dublin, Germany or France?

But the debacle on the Thames was all over the news this week because fundamentally all this talk of trade is too complicated and too boring to bother with. It might be crucial to our economic future, but it isn’t emotive or dramatic – at least until the factories and businesses start closing.

It astonishes me now how novel it seems to say that it doesn’t have to be this way. This could all just be sorted. If the Tory government cared about the fishing industry, or any industry, they would know that staying in the single market was the only option. The Brexiteers are intent on following their narrow ideology, for Britain to be a low-regulation tax haven at whatever cost, and they’ll have a laugh on a boat as they do it.

We in Merseyside, like much of the UK, still remember Thatcher’s disregard for British industry and the scars it left on our towns and cities. The Labour Party was built to stand up for working people, and this hard Tory Brexit poses a huge threat to jobs and living standards in our country. That is why Heidi Alexander and I convened the Labour Campaign for the Single Market, and why we are taking our campaign across the country to talk to Labour Party members. If we want a jobs-first Brexit, that means staying in the single market.

Alison McGovern is MP for Wirral South and co-chair of the Labour Campaign for the Single Market.

Alison McGovern MP, Lord Andrew Adonis, Cllr Ann O’Byrne and Erika Rushton will be speaking at the Maritime Museum in Liverpool tonight.