When anyone mentions fishing and farming to a Brexit minister, the usual answer is that they are a trivial 1% of Britain’s economy. Perhaps they are. But they are not 1% of its politics.

As talks start this week in Brussels, British negotiators have been sent into battle by Boris Johnson with the pompous rhetoric of Henry V at Harfleur. They must give not an inch. First up is likely to be fishing and farming.

As with fish, he faces the prospect of having de facto to nationalise an entire industry

These are totems of British trade with Europe – governed by decades, if not centuries, of rivalry, conflict and hard-fought compromise. European ships have fished British waters and traded British fish. If they are to be excluded and quotas torn up, as Johnson threatens, the EU has clearly stated that Britain can forget selling its fish on the continent. Indeed cross-Channel trade may be blockaded – the whole deal may be off.

The same applies to farming. Forty per cent of Welsh lamb goes to France, along with quantities of cattle and arable produce, itself now vulnerable to a block on migrant labour. Johnson reportedly promised farmers he would buy their sheep “into intervention” if farm exports were hit by a tariff. As with fish, he faces the prospect of having de facto to nationalise an entire industry. It is impossible to see any gain from this.

Fishing and farming are a high-profile corner of what has become a single, integrated continental market, much of it in time-critical goods. There is no way “deals with other countries” can compensate for its loss. Johnson’s talk of “driving a hard bargain” with Washington on US farm produce is for the fairies.

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Trade in foodstuffs has to be a matter of agreement and compromise. There need be no argument. Britain could simply agree to shadow Europe’s food standards for the purposes of its continued continental trade, leaving that trade frictionless as originally promised. Why should taxpayers have to pay for Michael Gove’s army of 50,000 border officials? Where is the gain?

Britain has left the European Union. National machismo is satisfied. It may be that one day the EU’s economy so collapses that it really is worth Britain’s while to turn away and seek better deals elsewhere. There is not the remotest sign of that at present. It makes obvious sense for Britain to remain in Europe’s collective Economic Area, single market and all.

When last under pressure over Ireland during the Brexit withdrawal agreement, Johnson the pragmatist was forced to capitulate. He now bellows about “prospering mightily” in the event of no deal. The inner man must know this is rubbish. Perhaps he should remember that careless talk costs votes.

• Simon Jenkins is a Guardian columnist