Devo Max – it sounds like a 99 per cent efficient toilet cleaner, or a dodgy West Country car dealer, but either way I visualise its initials in upper case. And that’s its problem.

It’s undoubtedly the ‘must use’ expression of the month. It’s not complicated, like ‘full fiscal autonomy’ or the Barnett formula, so anyone feels able to drop it authoritatively into even casual conversation. And everyone has their own idea of what it is.

For party leaders, desperate to save the Union in the final hours of the Scottish referendum campaign, it was perfect Looking-Glass, Humpty Dumpty-speak: it means just what we choose it to mean. Sign up now, check it out on the 19th.

For YesScotland campaigners it was a verbal Blob, impossible to pin down and attack – and especially frustrating, as they were the ones who had no need to check it out.

They knew its precise meaning because they’d invented it, and knew that it wasn’t at all what most wavering voters imagined they were being offered.

It originated in a 2009 Scottish Government paper, Fiscal Autonomy in Scotland. Five distinctive options were spelt out, ranging from the SNP Government’s preferred full fiscal autonomy (FFA) in an independent Scotland to a minimally changed current fiscal framework, which gave Scotland considerable discretion over spending but little over tax revenue raising, borrowing, or broader monetary policy.

‘Devolution max’ was the SNP’s fall-back option, clearly defined as FFA within the UK. The Scottish Government would be responsible not only for most of the public spending in Scotland, but for raising, collecting and administering virtually all revenues – instead of the estimated 15 per cent it would control even after the implementation of the tax-devolving 2012 Scotland Act.

The precision of that definition, as well as its content, makes Devo max entirely different from the third option of merely ‘enhanced devolution’, which really does sound vague, manipulable Humpty Dumpty-speak, and hardly surprisingly is unacceptable to the SNP.

Devo max, though, is not just definable. It can be viewed and studied in practice, for it broadly resembles the system in the Spanish autonomous communities of Navarre and the Basque Country.

Part of their autonomy is that the devolved governments are responsible for raising and collecting all direct taxes, including corporation tax, although, to conform with EU legislation and retain a harmonised social security system, indirect and payroll taxes remain centralised.

The two regions have used their powers to lower certain taxes below the rates elsewhere in Spain, thereby creating a relatively more competitive tax regime, which is, of course, also an SNP aspiration.

The problem, as noted by the 2009 Calman Commission on Scottish Devolution, is that Scotland – constitutionally, economically, as well as meteorologically – isn’t Spain.

A tax-based FFA might be operable in Spain, and conceivable in an independent Scotland. However, attempted within the UK, it would clash with the Treasury’s expenditure-based economic model and its pooling and redistribution of taxes to fund services and welfare.

Tax experts will argue that the devolution of some additional taxes – personal income tax, land and sales taxes, alcohol and tobacco duties – is perfectly feasible.

In other cases, though, for combinations of practical, legal and political reasons, it is less feasible, like the highly disputed oil and gas revenues.

In the UK, then, full fiscal autonomy short of independence is unattainable, and, even if attainable, would, as shadow communities minister Hilary Benn cautioned this week in Manchester, be effectively incompatible with the redistributive policies of our existing welfare state.

It would be incompatible too with the population-based Barnett allocation formula the three major party leaders committed themselves to. So, whatever additional powers Scotland eventually gets, they won’t amount to Devo max. In which case, why don’t we stop trying to appropriate the label rather meaninglessly for English local government, and instead work with the persuasive and realistic cases already being made by those with first-hand experience of running local authorities.

By all means, use Scotland as a benchmark – as in the challenge issued by Graham Allen MP, chair of the Commons Political and Constitutional Reform Committee: “I see no reason why English councils are not capable of taking on the powers that go to Scotland.”

And, certainly in Birmingham’s case, London too. The legislation is different, but the key recommendations of last year’s London Finance Commission are surely just as relevant and more straightforwardly implementable – particularly the proposed control over all property taxes: council tax, business rates, stamp duty land tax, capital gains property development tax, and the like.

It’s been good this week to see the County Councils Network, with its Plan for Government, 2015-20 and the Key Cities Group – 23 mid-sized cities, including Coventry and Wolverhampton – with its Charter for Devolution, determined not to have their distinctive voices and demands drowned out by the noise of the big cities.

There’s no question, though, that it’s in and around the big or the eight Core Cities where the devolutionary action is. Which should be good news for Birmingham, were it not for two big buts.

The first is obviously that the City Council has to survive Sir Bob Kerslake’s review in one piece. The second is that the most and the most meaningful devolution is happening where the big cities have been able to create Combined Authorities: Greater Manchester, West Yorkshire, Sheffield, Liverpool, and the North East.

Almost whatever the outcome of next May’s election, Combined Authorities are the future. So the big, if hardly new, question is: come the next parliament’s Big Glam Devolution Ball, attended by all the city regions, county regions, combined authorities of all shapes and sizes, which council names will be on Birmingham’s dance card?

* Chris Game, Institute of Local Government Studies at the University of Birmingham