If you asked any pro-Brexit campaigner in the first flush of euphoric victory for a snatch of poetry, the repertoire would have consisted of two items: one by William Shakespeare, the other by William Blake. Neither is what the triumphant English patriot thinks it is.

The first words to come to mind would have been John of Gaunt's evocation of a sacred England in Richard II:

This royal throne of kings, this scepter'd isle,
This earth of majesty, this seat of Mars,
This other Eden, demi-paradise,
This fortress built by Nature for herself
Against infection and the hand of war,
This happy breed of men, this little world,
This precious stone set in the silver sea . . .

There could be no more stirring evocation of Fortress England as a perfect “little world”. The imagined community of post-Brexit England is John of Gaunt’s demi-paradise, a place uniquely carved out by nature to protect its happy breed from the infection of foreigners.

But even as you read these lines you stumble over the word “isle”. Gaunt in this speech is talking explicitly about “this realm, this England” – not about Britain. And England isn’t an island. The speech keeps claiming the idea of a land surrounded by water, “bound in with the triumphant sea”. Gaunt’s geography is strangely out of kilter.

And so is Shakespeare’s history. John of Gaunt was Jean of Ghent, as in the city that is now part of Belgium. He was a French-speaking Plantaganet who spent much of his time in Aquitaine and became, for 15 years, titular king of Castile. When the actual people of England rose up in the peasant’s revolt of 1381, John of Gaunt was at the top of their hit list.

In any case, if you read the full speech it is not at all the hymn to English perfection suggested by its most famous lines. Shakespeare is actually making brilliant use of a rhetorical device, in this case antithesis: evoking something through what it is not.

The purpose of Gaunt’s hyperbole is to point up a contrast between this imaginary England and the actual place as it is under Richard’s reign: broke and mortgaged to the hilt. The imaginary island “bound with the triumphant sea” is in reality “bound in with shame”, awash with “rotten parchment bonds”. The speech is not a panegyric on England but a complaint about the country being in hock to bondholders.

The other poem was actually on the lips of the Leave campaigners last week. If they get the independent English state they desire Blake's Jerusalem will be its national anthem:

I will not cease from Mental Fight,
Nor shall my sword sleep in my hand:
Till we have built Jerusalem,
In England's green & pleasant Land.

But the adoption of Blake’s poem as a touchstone of nationalism is bizarre. It is a cry of protest at two pillars of English identity, the industrial revolution and the Church of England, both of which seem to be encompassed in its bitterly rhetorical question, “And was Jerusalem builded here, / Among these dark Satanic Mills?”

It embraces free love ("Bring me my arrows of desire") in the context of Blake's support for equality between the sexes. Its mystical vision of Jesus having visited ancient England is posed not as a claim to special status but as a series of disturbing questions. Like Shakespeare's "scepter'd isle" speech Jerusalem works by way of contrasts, evoking an English Utopia only as a rebuke to the present state of the country.

In its original form Blake appended to Jerusalem the biblical tag "Would to God that all the Lord's people were prophets." The poem is a call for a freethinking, egalitarian, democratic England, in opposition to the repressive and bellicose patriotism of the era of the Napoleonic Wars, when it was written. It's a fair bet that hearing it sung by Nigel Farage, Michael Gove or Boris Johnson would have made poor William Blake puke.

These ironies are worth pointing out because they underline something that may be important in the post-Brexit dispensation. Right-wing English nationalism is on the rise, but its cultural foundations are very weak. Even its top texts don’t mean what nationalists think they mean.

They both raise the most problematic question of backward-looking nostalgic nationalism: when, exactly, was this golden age of Englishness you want to return to?

At the end of the 16th century Shakespeare imagined John of Gaunt 200 years previously already complaining about how England had gone to hell in a handcart. Two centuries later Blake suggests that if you want to find the pure, holy England you have to go back to “ancient times” – and even then with a lot of question marks over their existence.

At the end of the “scepter’d isle” speech Gaunt says something that may in fact be more pertinent to England’s current situation that his earlier hyperbole: “That England, that was wont to conquer others, / Hath made a shameful conquest of itself.”