Shortly after midnight, an email arrives. From a leading light of the campaign for Scottish independence, one who six months ago dreamed of nothing better than a decent showing for yes and an honourable defeat. It says simply: “This just might be happening.” That message came on a fevered night when political chatter centred on a rumoured poll showing a small lead for yes, perhaps one of several such polls coming on Sunday. If that happens, says one Edinburgh sage not prone to hyperbole, “it will set the forest on fire”.

The pro-union camp can smell the smoke. It has seen the reports of midnight queues at council offices, as never-before voters demand they be registered in time to have their say on 18 September. They can see the blue and white placards and badges proliferating on every surface, they hear the talk of record-breaking turnout – and they fear the energy, the momentum, is for yes.

A sure sign of a campaign that believes it’s facing defeat is when the recriminations begin, the post-mortem conducted before the patient is pronounced dead. That is not happening – yet – on the unionist side, but I felt the first breath of it in a conversation this week with one of the noes’ biggest hitters. “The Conservatives have fucked this up from beginning to end,” he told me. You can’t sell to Scots a Scotland forced to live under cuts and privatisation, slashing the top rate of income tax and imposing the bedroom tax, he said. You can’t do all that and then turn to the referendum as if it were a separate, purely constitutional matter. Put another way, none of this would be happening if there were a Labour government in Westminster. It’s the chance to break free of a Britain moulded in the Tories’ image that is luring Scots – or at least a critical slice of traditional Scottish Labour voters – towards yes.

Others say it’s the no campaign’s consistent negativity that’s made a yes victory possible. The noes should have begun with a message of warmth – a spirit-of-2012, Danny Boyle-ish celebration of all that Britain has been and could yet become – only turning negative in the closing weeks, to seal the deal. Instead, Better Together went negative from the start – shaking their head and saying this or that will never work – so that now the no campaign finds an electorate numb to its warnings. What’s left for the final push, that period when – if the Quebec referendum of 1995 is a precedent – an emerging yes can be converted into a nervous no by sowing last-minute doubt? Another warning of economic ruin from a bank or mega-corporation? The Scottish public are inured to such things now.

As a matter of tactics, Better Together should never have agreed to fight this battle on ground of the nationalists’ choosing. Of course, Alex Salmond had the key advantage: he got to write the question on the ballot paper. But Alistair Darling has conceded on the decisive matter of language. He freely says he is against “independence”. The word should not pass his lips. He should insist he is against “separation” or “break-up”.

What are those of us watching from afar to make of all this? How should Britons outside Scotland, especially those on the centre-left, react if the polls keep tightening and Scots vote yes? Some are eyeing events with envy, jealous of the awakening under way across the border: the packed public meetings, the debates in pubs and on street corners, the animation of civic life that the referendum has brought to Scotland. And it’s not just the engagement they covet, it’s the chance to cut loose from a solar system in which the City of London is forever the sun. For them, Scots’ chance to say yes seems nothing less than thrilling.

I understand that feeling. I’ve written before that were I in Scotland, I too might vote yes. But I’m here, several hundred miles away, and now that the prospect of independence is clear and present, my reaction is different. When I contemplate the prospect of waking up on 19 September to discover the union has been defeated, I can’t help but feel a deep sadness. I know that this is for Scots to decide, that any member of a union always has the right to secede. But just because it’s their choice doesn’t mean the rest of us are not allowed a reaction. And if that reaction is emotional, that’s allowed too: the business of nations and nationhood is nothing if not emotional.

This decision of the Scots will affect every Briton outside Scotland. Our country will change. At its most basic, a yes vote will mean that, at a stroke, the UK will lose a third of its land mass and close to a tenth of its people. The mountains and lakes of Scotland will still be there, of course, but they will be the terrain of a foreign country. They will no longer be part of our shared inheritance. I am haunted by the words of the Czech who remembers the sensation when his country no longer included Slovakia: “It felt like an amputation.”

‘British” will become an extinct term, too baggy and ill-fitting for the rump UK left behind. The English will account for more than 90% of the population of this leftover entity, while the Welsh and Northern Irish huddle together making up the rest. We will have to let go of British and Britishness, terms long mocked for their vagueness but useful all the same. Not least for those of us from minorities, who have found living in a country defined by its very plurality, a composite of four nations from the start, easier than in most places. “British” works well next to an unseen hyphen – black British, Muslim British, Jewish British. But if Scots vote yes, we will have to learn that trick anew alongside the word “English”, a category whose history is not quite as generous.

Put aside the electoral calculus that shows it will be harder for Labour to form a government in Westminster without Scottish seats. It goes deeper than that. Wasn’t the 21st century vision meant to be one of interdependence, with nations working together? Surely it’s the Tory right and Ukip that insist on exclusivity of sovereignty, adamant that it can never be shared. Until now, Britain has represented a rare experiment in shared sovereignty, pooling risks and resources across borders. But a yes vote will end all that, declaring it a failure.

So I understand the exhilaration both inside and outside Scotland as yes appears within reach. But from where I stand, on this side of the border, I find myself hoping Scots don’t give up on this odd, messy, imperfect union just yet. And I admit, part of that is selfish: I worry about life in the country they’ll leave behind.

Twitter: @Freedland