How should we “take back control”? Either we have to find a way to agree on new rules of engagement with the EU and the rest of the world (in which case, we won’t really be “in control” after all) or, instead, we have to disengage.

There were hints of the latter in Jeremy Corbyn’s speech to the Engineering Employers’ Federation last Tuesday. The Leader of the Opposition wants to “Build It In Britain Again”. Admittedly, he doesn’t want to “crash out” of the EU, partly because he has no desire to cosy up to Donald Trump. He does, however, want to make sure that a future Labour government — via its public procurement policies — would “buy British”. Apparently, autarky is best.

In Corbyn’s words, “We will build things here again that for too long have been built abroad because we have failed to invest… we want to make sure the government uses more of its own money to buy here in Britain.” He cites the example of train carriages (not surprising, perhaps, given his experience of sitting on their floors): “We have plenty of capacity to build train carriages in the UK and yet repeatedly over recent years these contracts have been farmed out abroad, costing our economy crucial investment, jobs for workers and tax revenues.”

And you might think he has a point. Yet, when it comes to manufacturing, life can be terribly complicated. A quick look on Hitachi Rail Europe’s website, for example, tells me that Newton Aycliffe in Co Durham, which provides 730 jobs and is capable of producing 35 vehicles per month, is already in the business of building trains that will serve ScotRail​ , the East Coast Main Line and the TransPennine Express. It also intends to produce trains for use elsewhere in Europe.

Then there’s Construcciones y Auxiliar de Ferrocarriles (CAF), the Spanish train manufacturer, which has just opened a state-of-the-art facility in Newport, Wales. It’s constructing trains for the Wales and Borders network and other operators up and down the country. Bombardier — headquartered in Montreal but with operations in Derby — received orders earlier this year to build additional trains for the Elizabeth Line (as Crossrail has now become). According to its website, “Bombardier is the only company that both designs and manufactures trains in the UK.”

Meanwhile, Siemens Mobility, the German train manufacturer that is presumably responsible for Corbyn’s claim that “if you go to Germany you’ll struggle to find a train that wasn’t built there”, is in talks to merge with Alstom, its French rival: if the transaction goes through, the new company would be headquartered in Paris. The proposed deal is in response to heightened Chinese competition. Better, it seems, to be a European heavyweight than a national champion.

All of this raises a rather tricky philosophical question. What does it mean to “Buy British” (or, for that matter, to “Buy German”)? Let’s take Mini, a world-famous British brand. The cars themselves are assembled in Cowley, a stone’s throw from Oxford. Yet, of a typical Mini’s components, many hop across the Channel on multiple occasions.

Mini is, of course, owned by BMW , a company headquartered in Germany. BMW’s institutional equity investors, however, come from far and wide. Remarkably, of BMW’s total institutional equity, a bigger share is owned by British investors (15.9 per cent as of the middle of last year) than by their German counterparts (15.1 per cent).

"Hitachi Rail Europe is already building trains that will serve ScotRail and the East Coast Main Line"

Corbyn states that “Labour will look at how to further support [small and medium-sized enterprises] to participate in the tendering process instead of them being dominated by faceless multinationals.” Yet it’s precisely those faceless multinationals — from Japan, Spain, Canada and Germany — that are responsible for much of the UK’s manufacturing success. “Buying British”, it turns out, is not quite as simple as it seems.

The Conservatives, meanwhile, are caught between two versions of the “new rules of engagement” approach. The first is reflected in the Brexit White Paper . We’re leaving the EU but we plan to remain in a bespoke customs union with our near neighbours. Among other things, that should ensure frictionless trade between Northern Ireland and the Republic. The obvious problem with this arrangement, however, is that we cannot so easily negotiate trade deals with countries elsewhere in the world. And there’s also the tricky issue otherwise known as Michel Barnier.

In the second version, we are no longer in an EU customs union but we are, apparently, in control of bespoke trade negotiations with the US, China, India and Brazil, among others. Yet it’s difficult to believe that we can “tell” the Americans what a bilateral trade deal should look like, particularly after last Wednesday’s trade bromance between President Trump and Jean-Claude Juncker , the President of the European Commission.

The Chinese are not likely to be particularly receptive either. Indeed, knowing how desperate the UK would be to strike post-Brexit trade deals, it’s more likely that our trading relationships would be under others’ control, not ours. And, of course, this arrangement makes the Irish border a particularly thorny issue.

So which version of “taking back control” do you want?

Mr Corbyn’s “Buy British” campaign is both uncomfortably Trumpian and too redolent of the UK’s earlier industrial failure (don’t forget that the horrific Pacer train, still in use today, was effectively a converted bus on bogies). The Tories’ approach, meanwhile, is based on wishful thinking: we cannot easily have a close economic relationship with the EU and, at the same, time, the freedom to negotiate trade deals worldwide. With either approach, “faceless multinationals” could choose to go elsewhere, making us all worse off.

Of course, we could always hark back to the days of empire when a well-placed gunboat would properly establish British “control”. I’m hazarding a guess, however, that imposing our rules on everyone else might not be greeted with a great deal of enthusiasm.