It was 1977 when Tom Nairn spooked the political world with his famous book, The Break-Up of Britain. He predicted Scottish independence, a bit prematurely. But last Tuesday, as Alex Salmond and Nicola Sturgeon launched their government's manifesto for an independent Scotland, ancient Britain's citizens were being offered a break-through as much as a break-up.

In itself, the fat policy manual isn't revolutionary. Scotland's Future is a sturdy, sensible, well-written catalogue of aspirations – all of them achievable with luck and skill. But what's so exhilarating is the flock of many-coloured hopes gathering behind this project, like seabirds in the wake of a working trawler. Scotland's departure from the union could mean all kinds of liberations and reinventions for the islanders who live under the crown.

England, above all, could at last disinter its identity and the buried radicalism of its people. Stripped of the "British" comfort blanket, the archaism of England's power structure and its monstrous north-south imbalance would become visible and intolerable. And in Scotland itself, there would be a violent climate change in politics as parties ceased to be London's branch offices.

Scotland is in many ways a naturally conservative country – with a small c. A new rightwing movement, freed from association with "down south" posh boys and Maggie Thatcher, would find strong support. More significant, there would be an insurrection in the Scottish Labour party. With a fresh leadership committed to using independence for social justice, I'd expect such a party to push the Scottish National party aside and form Scotland's government within a few years.

Then there's the factor of opportunism, comically familiar to small countries. I have seen it in Scotland before. When whiffs of independence spice the air, the big Union Jack men talk differently down the telephone at night. "Of course I can't say this openly, Jimmy, but I want you to know that if it comes to it, I've always been privately …" Lawyers, bankers, union leaders and unionist leaders – they'll realign in droves "if it comes to it". Why not?

Much the same applies to the apparently fearsome rebuttals to Salmond's document. On inspection, they are nine parts bluff. What makes cheeky Salmond think an independent Scotland would be allowed to use the pound, or enter the EU, or be admitted to Nato? Well, the answer is another question: "if it comes to it", what sort of Scotland do you want as a neighbour? Does London seriously want to force a currency frontier at the border and screw up trade with England's second biggest partner? Does Brussels really want to expel a loyal member and accelerate the EU's disintegration? Does Nato want a new hole on its east Atlantic flank? No, if the Scottish people do vote yes in September (which is still unlikely), healthy opportunism will cobble up solutions to all these problems.

Reading Scotland's Future, I couldn't at first account for a faint twinge of melancholy, a recognition. Then it dawned on me. The Scotland being here described – or proposed – was the Britain so passionately hoped for by the millions who voted for Tony Blair, back in 1997.

After 18 years of Thatcherism, the longing was for a return to fairness and a stronger regulating and redistributing role for the state. What New Labour did with those hopes is another story. But Salmond's "what sort of Scotland" is also a moderate, statist social democracy that partners the private sector but is not afraid to – for example – renationalise the Royal Mail.

The yes camp is wider than the official yes campaign. Around Scotland in recent months, I keep meeting people who would never vote SNP or trust Salmond, but who are painfully admitting that they may have to vote yes. This is because they are appalled at the way the British state is heading, under Tory or Labour: the downward plunge into the barbarism of neoliberal politics, the contempt for public service, the almost monthly advance of privatisation. Wrestling with old loyalties, they may vote for what Ian Jack called "the lifeboat option" – an independent Scotland as the only way to escape that fate.

It's a lifeboat the SNP government has already launched, using devolution to keep out English "reforms" to the NHS or higher education. Gordon Brown himself used to argue that the health service and the postwar welfare state were the supreme achievement of Great Britain's history. And yet it's only the SNP that has embarked on this astonishing attempt to preserve and grow what's left of that achievement in one part of old Ukania. It hurts to laugh at some of history's jokes, but here's one: in spite of itself, the SNP is the most truly British party in these islands.