The National Gallery’s colonnaded splendour radiates across Trafalgar Square a sense of the importance of art in Britain’s national life. But the reality inside is far less glorious. The 400 gallery assistants are about to be outsourced to a private company against their will, to squeeze pay and conditions. A ballot by their union, PCS, closes this week, calling for a five-day strike in protest.

All day they guard the nation’s treasures: not automata, but well-informed, if untrained, guides who like to be asked questions, know where paintings are, are glad to advise nervous visitors unsure what to look at. Some have worked there for 40 years, some are younger, many former art students. Ben, who has been there for over a decade since art school, is an artist when not at work stopping people poking the paintings. His favourite beat is the impressionists and Van Gogh, where he sends the ditherers: all assistants have their preferences, he says (Poussin and the French classicists being the least popular).

A letter to staff from the director, Nicholas Penny, says all gallery services go out to tender in April, something no other national gallery or museum has done. That includes visitor services, school bookings, public information and even complaints. As Tupe – Transfer of Undertakings (Protection of Employment) – regulations require, staff will transfer to a private company on the same terms, but that’s weak protection: they can be sent to work anywhere in that company. As one fearful gallery assistant says: “I came to work at the National Gallery, but I could be transferred to a supermarket car park.”

This is the only national museum in London not paying the living wage. A tangle of pay rates means that older staff (predominantly male) are paid more than newer (predominantly female). But even if men and women’s pay were equalised, many of the older staff would still lose out. The gallery’s grant is being cut, so it needs to make more money with extra paid evening events. Staff are willing – but expect extra pay. Management says negotiations went nowhere, so they have to go nuclear: let a private company get tough with them. Some would leave: they note that the security firm CIS, brought in to staff the recent Rembrandt exhibition, took away the chairs used by the gallery’s own guards. That’s how private companies profit from these contracts: as old staff leave, new staff can be hired at any pay rate.

One gallery trustee I spoke to admitted that CIS had been brought in to run the Sainsbury wing to give the gallery staff a fright. CIS, if you look at its website, mostly provides heavies to guard empty buildings, not talk to the public. Most trustees, I’m told, would like to keep the staff in-house.

What happens in hard times is always the same: spreadsheets show the most crushable item is staff. Numbers are cut and squeezed hard for longer hours. Vital breaks in a long day are eliminated. Changing into your uniform or taking your tea break has to happen on unpaid time. It’s easier to let ruthless companies to do the dirty work, so squeamish managers can wash their hands of consequences. Both sides will now go to Acas: there is still time to pull back from this privatisation too far.

This shedding of long-term employees is emblematic of low-pay Britain, where a million public jobs are being lost. Public servants are more unionised than other workforces, and so irksome to managers who eye a commercial world of 19th century employment practices – lump labour on zero hours, temp agencies, free interns, the bogus self-employed free of national insurance. The public servants’ ethos, their attachment to the civic realm, has been systematically trashed as mere unionised self-interest.

What’s afoot at the gallery explains why pay is falling as a share of GDP. Galloping inequality is the result of a million such decisions employers think prudent, mainly because everyone is doing it. The derelict Low Pay Commission has let the minimum wage itself fall £1,000 in real value since 2008. The Tory call last week for higher wages was breathtaking dishonesty, echoing the TUC’s “Britain Needs a Pay Rise” campaign. The government has huge sway over pay. If it demanded the living wage not just for its employees, but also from every contractor and supplier, then national pay norms would rise instantly. If the Low Pay Commission set higher minimums appropriate to each sector, like the old wages councils, larger companies could pay decent salaries according to profitability.

The Joseph Rowntree Foundation’s latest report finds 40% of families with children living below a minimum threshold of decency. Most are in work – but earning too little to buy what a majority of the public in focus groups consider essentials for participation in society: no drink or cigarettes, £5 a fortnight for eating out, £40 for Christmas food, one week’s UK holiday.

That takes an income of £20,400 a year for a couple with two children – which is almost £3,000 more than National Gallery assistants earn. GDP growth is near invisible to the 8 million squeezed hardest, by this government’s deliberate choice: average income loss since 2010 is £33 a week. Now the government promises full employment – but what kind? Most new jobs are low paid, precarious and part time. For all George Osborne’s hi-vis posturing about his sham “northern powerhouse”, the latest figures from the Centre for Cities show how far the gap between the south and the rest has widened.

At Davos this week, central bankers will echo the fashionable view that accelerating inequality is the real economic risk, a danger to capitalism itself. Oxfam tells them 1% of the population will by next year own more of the world’s wealth than the other 99%: in Britain, the top 1% has soared away, and the bottom 10% has done worst while paying the highest proportion of its income in taxes: 47%. But no sign yet that the Davos set is worrying unduly: by Epiphany – 6 January – FTSE 100 chief executives had already earned more than a year of the average wage.

How do you wrest back wealth from them? Restoring power to unions would help, ensuring every workplace is offered union membership. Instead, Cameron’s manifesto will make strikes near-impossible, with a 40% ballot threshold unknown anywhere in the democratic west. That’s a reason to hope the ever-patient attendants at the National Gallery resist being cast out to G4S, Serco and the rest– and remain as treasured employees of us all.

• This article was amended on 20 January 2015. An earlier version said: “Oxfam tells them 1% of the population will next year own 99% of the world’s wealth”. This has been corrected to: “Oxfam tells them 1% of the population will by next year own more of the world’s wealth than the other 99%”.