SUPPOSE, just suppose, that half a million people or more marched through the centre of London on Saturday demanding a break-up of the British state and the election of an independent English parliament.

Let’s further suppose the sun shone and the happy, good-natured demonstrators waved a sea of picture-friendly English flags bearing the cross of St George. Imagine, too, that the protest reverberated to the sound of the bells of hundreds of Morris dancers celebrating their national culture. And of course, there would be lots of excited, tail-wagging dogs draped in red, white and blue.

Such an event would be of seismic political importance, given Brexit. Breathless BBC correspondents – they seem to have one for every conceivable topic – would be busy on the box reassuring one another that Theresa May could no longer afford to ignore Middle England. The BBC Home Counties editor would quote the BBC Morris Dancing correspondent to the effect that never had so many English folk dancers been seen on a political protest since Oliver Cromwell banned Whitsun.

On the Andrew Marr Show, Jacob Rees-Mogg would tell us this was a warning that ordinary Britons would not be taken for granted and that the Moggy was ready to roll up his trouser legs for England. Every Sunday newspaper would carry a picture of the demo on its front page. The Mail on Sunday headline would thunder: “Goodbye Europe, Hello England!” and they would run six inside pages of photographs of middle-class Essex girls in straw boaters emblazoned with ribbons declaring: “I love Jacob”.

The Sunday Times front page would be more cerebral: “Biggest ever spontaneous demo calls for English freedom. May calls emergency Cabinet meeting.” And, come Monday morning, an acerbic (and very Welsh) John Humphrys on the Today Programme would ask Jeremy Corbyn why he was “more interested in Venezuela than in the authentic voice of England’s Silent Majority?” Humphrys would then interrupt Jeremy before the latter could get a word in.

But this is a mere daydream. Instead, on Saturday, upwards of perhaps 80,000 people spontaneously marched for Scottish independence through the surprisingly sunny streets of Glasgow, which was once proudly referred to as the Second City of the Empire. No political party had called this demo. No major politician had orchestrated its publicity or timing. This was a people’s march, if ever there was one. There were Saltires by the veritable thousand – and not a few Senyera, the nine-barred Catalan standard. There were even flags fusing the Scottish and Catalan banners – a sign that internationalism is alive and flourishing north of the Border.

How many marched on Saturday? The police – never reliable – reckoned 35,000. If so, that was more than at any previous independence demo. If the numbers were between 50,000 and 80,000 – as the organisers and most participants seem to think – that would represent a massive turnout for a political protest of any kind in Scotland. Scaled up to a UK figure, that would represent well in excess of half a million souls. And if Saturday’s march was closer to 100,000, we would be in territory that would prefigure a political crisis anywhere else.

But as we all know, Saturday’s independence march in Glasgow rated but as a minor footnote in the Brit media. The reporting that did take place was at pains to avoid any deep analysis of what provoked the indy demo, or what it portended locally. Instead, on Sunday morning, Andrew Marr chose to interview Arlene Foster, the leader of the DUP. Scotland rated no mention. Typically, Andy treated Foster with kid gloves. Imagine his line of questioning had 80,000 Irish Republicans marched through Belfast on Saturday demanding a united Ireland instead of Brexit. The curious element in this predictable media amnesia regarding Scottish independence is that it is a deep-seated reflex. The Brit media – from the rabid red-tops to Murdoch’s pretend “quality” papers – simply believes that Scotland and Scottish political aspirations are not a real story worth covering.

Nobody at the BBC in London wrote a memo to order the newsroom to ignore a massive protest in Scotland of folk who want to exit the UK. It is simply ingrained in the Oxbridge-educated BBC elite that Scotland is a news irrelevance.

But why this cultural bias? After all, a mass demo aimed at the heart of the British state is hardly lacking in basic news interest.

The answer is that deep down the British media – indeed, the whole London metropolitan intelligentsia – have a pathetically neocolonial mentality. This is reinforced by the concentration of media ownership and control in London. As a result, the London elite never see a political express train coming their way till it hits them. That goes for the Windrush debacle and it goes for Scotland, which they have simply written off as “no longer a problem”. It is imperative that we break this near monopoly of thought control by London. Creative Scotland, for instance, needs to direct more resources into local publishing and television. It is also vital we develop existing initiatives in internet broadcasting to replace the BBC. Scotland is an entrepreneurial nation – surely it can create exciting broadcasting outlets to eclipse the tired, unimaginative output of Aunty Beeb?

Saturday’s march is also a wake-up call for the ever-cautious leadership of the SNP. The size of the protest was prompted by two developments. First, the growing anger among independence voters that the Tory Government is using Brexit as a means to grab back powers from Brussels that rightly belong at Holyrood. Brexit is going to put huge pressures on the UK economy which means a massive attack on incomes and workers’ rights. Westminster needs to prepare for that by re-centralising as many economic powers as it can while happily ceding welfare spending bills to the devolved parliaments.

That message has to be hammered home by the SNP and the wider movement: independence is an urgent necessity to forestall both a power grab by London and a second round of austerity.

Secondly, Saturday’s fabulous turnout came as a result of the grassroots indy movement taking the initiative rather than relying on politicians to take a lead. Call this the Catalan Effect. The Catalan independence movement has been driven by popular mobilisations organised by the Assemblea Nacional Catalana (ANC), the non-party umbrella for local indy groups and individuals.

The First Minister is reticent about calling another independence referendum until after the Brexit deal is clear. I understand that, but it rather leaves the indy movement in limbo. Yet we need to get the independence message across to the nation ASAP.

Saturday’s powerful turnout must be followed up by fresh, pro-indy popular initiatives. A Freedom Chain linking hands across Scotland from Dumbarton to Dunbar? A national day of “teach-ins” at every university and college to discuss what a future indy Scotland could look like? A mass sit-down (Catalan-style) to close the M8, to protest Westminster’s power grab?

These initiatives must lead to the emergence of a similar body to the Catalan ANC, capable of driving the movement forward. This is an urgent agenda as a second independence referendum cannot – and must not – be far off.