“The Mother of Parliaments has been shut down by the father of lies.” So thundered Aidan O’Neill QC over Boris Johnson’s unlawful prorogation of Parliament last year. Now he’s claiming that the Scottish Parliament can ignore mother and hold its own referendum on independence.

Hitherto, Nicola Sturgeon has insisted on going down the conventional route of asking Westminster’s permission first. But now that that permission has been refused, many of her followers will be wondering what she’s waiting for. Take Boris to court, FFS. Send in the suits.

The First Minister now arguably has a plausible Plan B. But she is most unlikely to use it, even though her other options are nil.

Boris Johnson’s terse rejection on Tuesday of the First Minister’s request for a legally authorised referendum was hardly unexpected. Once in a generation ... time to end divisions ... get on with the day job.

There was no hint of concessions on powers for the Scottish Parliament, let alone moving towards a federal constitution – something the Prime Minister has talked favourably about in the past.

This was the “big flat No” to independence that Michael Gove promised during the General Election campaign. It’s a door slammed very publicly in the First Minister’s face.

Yet David Mundell, the former Tory Scottish Secretary, had said only last August that the UK Government would “have to listen” if the pro-independence parties won a clear majority in 2021. But Boris is not in listening mode. So what happens now?

Nicola Sturgeon made the usual noises about Scotland’s “right to choose”, echoing the rhetoric of 1970s abortion campaigns. She insists, all evidence to the contrary, that a referendum is now “inevitable”.

READ MORE: Gordon Brown: UK will have to change if Scotland is to stay 

The Constitution Minister, Mike Russell, huffed and puffed as only he can. Johnson can’t stand against the people’s will indefinitely, he averred. But he gave no suggestion of any kind of action, legal or otherwise.

This is because his boss, Nicola Sturgeon, is committed to the Section 30 order. This means recognising that the UK Parliament is sovereign and asking the PM to transfer power to Holyrood to hold a legally binding referendum.

This is the only way, she says, to put the result “beyond legal dispute”. And more importantly to ensure that the referendum is recognised by other countries, in particular, the EU.

Most legal folk, from the prolific law lecturer Andrew Tickell to the constitutional law expert Professor Aileen McHarg – both sympathetic to the nationalist cause – agree with the First Minister.

As McHarg puts it: “There is no legal shortcut to independence.” Tickell says it would be a waste of money mounting hopeless court actions. But now O’Neill has thrown a hand grenade into what was an arcane dispute.

Financed by the Forward As One group, this senior advocate’s considered view is that the Scottish Government already possesses the power to call a consultative referendum. He thinks the matter should be tested in the Court of Session and thinks that judges might issue a “declarator”. The implication is that if the Scottish Government has a will, it could find a legal way.

Now, it may be that the Scottish Parliament has the power, in the abstract, to hold referendums. Holyrood could have binding referendums on all manner of things: smacking, speed limits, hospital charges.

Anything that is within the powers of the Scottish Parliament, in fact.

But the point is that the constitution is not among the powers of the Scottish Parliament. It can stage a ballot, but what it can’t do is force Westminster, ie Boris Johnson, to honour the result of it. At least that is the majority view.

READ MORE: Kevin McKenna: Strategy, not lines in the sand, will set Scotland free 

Under the last Parliament, there might have been a slim chance that the opposition parties could have ganged up to force the PM’s hand. They might have passed a Benn Act instructing the Government to honour a Scottish referendum result. But Boris Johnson now has an 80-seat majority, so there’s not a cat in hell’s chance that Westminster would agree to Scotland seceding from the Union. Which raises the question of what to do next.

Alex Neil, the former Scottish minister, says Sturgeon should press ahead anyway. So what if Westminster ignores the referendum result, he says. It would stand as a measure of the Scottish people’s will. “The political argument would be,” he said on BBC radio, “we’ve won.”

It is going to be very difficult for the First Minister to disagree. But that is probably what she’ll do. Her argument has anyway changed recently. She is not actually arguing for an immediate referendum any more, but for Holyrood to be given the power to call a referendum in future.

That is harder for Boris Johnson to reject, but reject it he has. No referendum for now or in the foreseeable future. Do your worst. It’s not going to happen.

If Sturgeon takes the O’Neill advice and holds a consultative referendum, the UK Government will reject it as invalid. The non-nationalist parties will advise unionists to boycott it, as unionists did in the 2017 Catalonian referendum. This would fatally undermine its legitimacy.

Nationalists might say: a result is a result. Why bother about people who won’t vote? What about the force of democratic opinion? But it is naïve to believe that Westminster parties will respond positively to any unauthorised referendum.

Earlier this week, Lisa Nandy, one of the leading Labour leadership contenders, told The Andrew Neil Show that the UK should look to Catalonia for lessons on how to beat “divisive nationalism”.

We are assured that Nandy did not mean that riot policy should be used against peaceful demonstrators or that elected Scottish politicians should be locked up. But it was still an astonishing remark.

READ MORE: Alistair Grant: Scotland's endless Indyref2 Groundhog Day is going nowhere fast 

Post-Brexit, metropolitan attitudes have hardened markedly against independence and “divisive” nationalism. Nicola Sturgeon will eventually have to recognise that the Westminster route to independence is barred for the foreseeable future.

The precedent of the 2012 Edinburgh Agreement will not necessarily be recognised following Brexit. Britain’s departure from the EU has changed the game.

In 2014, no-one thought the UK would be leaving the EU. Scotland and England would be remaining in the European Single Market and the Customs Union even after independence. Not any more. There would now be the question of a hard border at Gretna.

But more importantly, the Global Britain project would be fatally damaged if the UK were to lose one-third of its landmass, most of its hydrocarbons and all its nuclear weapons. It’s just not gonna happen. We are in a different era.

The UK state can no longer afford to endorse Scotland’s departure from the Union. Even Labour is now locking the door. This means there is not going to be a legal indyref2, possibly for a very long time. Not after 2021, probably not after 2025.

Brexit Britain will not let Scotland go, unless there is overwhelming support for independence, which frankly there isn’t. Nicola Sturgeon may insist that the will of the people will triumph in the end. That there is no point in holding referendums that don’t mean anything.

But she can’t in all honestly keep telling her troops that indyref2 is round the corner. It isn’t.