London (CNN) The makeshift poster is displayed proudly in the window of a London home. "We love the NHS" is written in children's handwriting above a picture of a rainbow. A few doors down the street, another colorful arch is daubed on a bed sheet hanging from a balcony.

It's a common sight in the UK these days. Britons have been publicly declaring their love and support for the National Health Service , or NHS, by adorning their windows with pictures of rainbows. At 8 p.m. every Thursday for the past four weeks, they've opened their windows to clap and bang on pots and pans , to express their gratitude for the health workers fighting the country's coronavirus battle

The Covid-19 pandemic has brought the cherished national institution known by its three-letter acronym even further to the center of British life. Chronically underfunded and stretched to its limits even during normal times, the health service has never been under so much pressure. It has also never received more praise, love and gratitude from the public.

But that support is nothing new. In public opinion surveys, people over and over again identify the NHS as the one thing that makes them most proud to be British, well above the royal family . When London hosted the Olympic Games in 2012, the opening ceremony featured an entire musical number honoring the NHS and its staff, with nurses dancing around children in hospital beds.

"There is this huge level of deep-rooted affection for the institution and pride in it and what we're seeing now is that affection being manifested," Dan Wellings, senior fellow at the King's Fund, a healthcare think tank, told CNN.

It's hard to pinpoint an exact reason for this affection. "It's not in data or numbers and evidence, it's feelings and emotions and all the things that people can't explain very well," said Laura Duffell‏, a matron at King's College Hospital London. Duffell‏ works for the NHS and said she "wouldn't work anywhere else."

"It feels like you're part of a family in all the different hospitals and all the different teams that I've worked in, the team spirit is always there."

The government is tapping into those feelings to reinforce its strict social distancing measures. That messaging is clear: Stay at Home, Protect the NHS, Save Lives. The slogan is on every information leaflet the government has printed. It's even on the lectern from which the country's top officials -- standing in for Prime Minister Boris Johnson who is himself recovering from Covid-19 -- deliver their daily updates.

The NHS is not unique. Many other European countries have publicly funded health systems that provide free medical care for everyone. But none can claim the NHS's level of appreciation.

"The NHS is seen as something that we all have in common, something that we need never question, something we don't need to plan for," said Cal Flyn, an author and journalist. Flyn collaborated with The Wellcome Collection, a museum and library dedicated to health, on a major project marking the NHS's 70th anniversary in 2018 . "The fact that we don't need to pay for health insurance, and yet it is always there for us, especially at our lowest moments, makes it very morally unquestionable. It is seen as a force of pure good," Flyn added.

The origin story

The NHS was founded in 1948 as part of a huge national rebuilding effort following World War II. Since then, it has become an integral part of British society and the country's biggest employer.

It's partly its origin story that makes the NHS so important to many.

"There's a sort of folk memory of it ... people wanted a really big change and the NHS was part of it," John Appleby, the director of research and chief economist at the health think tank the Nuffield Trust, told CNN.

Wellings added that the sense of post-war collectivism is still a big part of how people view the NHS. "There are some values that are attached to it which play very strongly to a sense of Britishness: fairness, available to all, it's equal, it's primarily funded through taxation and it's free at the point of need." The branding is great too. "It's so clear. The fact that it says 'national,'" Wellings said.

The NHS, everyone agrees, is part of Britain's national psyche.

"Stay home, protect the NHS, save lives" has become the central coronavirus message in the UK.

The key principle of the service is simple: medical care should be free for everyone. Whether it's a routine check, chemotherapy or a hospital birth, patients do not need to worry about pulling out their credit cards when getting treated. A few parts, including dentistry, optical care and pharmacy have been privatized over the years, but the bulk of the service is public. Prescription drugs are free, but most people need to pay an prescription charge of £9.15 ($11.45).

"Virtually everybody contributes to the NHS in some way, there is no special NHS tax, it is not just funded out of income tax, it is funded out of all taxes, so everybody is putting in something, and the deal is, no matter who you are, whether you're the Queen or me, there's equal access to the things you need when you need them, and it's decided not on your income but on your healthcare needs," Appleby said.

Funding issues

After the 2008 financial crash, the government's tax revenues have suffered, and so did NHS funding. Health think tanks such as the King's Fund say the NHS is now particularly vulnerable, as under previous Conservative governments, its funding did not match the increasing demand for healthcare. That led to longer waiting times, reduced availability and staffing shortages, according to research by the think tank.

Attempts at injecting private investment brought short-term gain for long-term pain. One scheme used by Tony's Blair Labour government to fund big NHS projects using private money cost a fortune: according to Nuffield Trust, by 2049, the NHS will end up repaying £82 billion for just £13 billion worth of such projects.

Many Brits are acutely aware of the pressures the NHS is facing. When the call came for volunteers to help with the current crisis, nearly 1 million people signed up. There's also been a huge spike in donations to charities that support the NHS and its staff.

But not everyone is comfortable with the idea that the NHS should rely on charity. "The response to these fundraisers is a little bit split, depending on one's politics ... if you're more left wing, you feel there should be no question that this money should come from central government ... at the same time, people want to help, and they want to treasure the service and to keep it running, especially in a time of crisis," Flyn said.

In the UK, the NHS seems sacred and the resistance to overhauling it in a major way cuts across the British society -- most people said they would pay more tax if the NHS needed it, according to the surveys.

"People would not put up with politicians messing around with the very basics of the NHS," Appleby said. The promise that Brexit would mean more cash for the NHS was one of the decisive factors in the UK's referendum on European Union membership in 2016, even as many economists warned that leaving the EU would likely hurt the UK economy and put NHS funding under pressure.

As the grim reports of death and suffering keep coming in, day in day out, many in the UK are looking to the future, hoping the crisis will bring change to the way the NHS and its staff are treated.

"But perhaps on the other side of the coronavirus it will give them, their unions and so on enough ammunition to campaign for more money. I think there would be a lot of public support for that," Flyn said.

Duffell, who is active in the RCN, the nursing union, is less optimistic. "I would love to see the government investing in the National Health Service to the extent it should, but I just don't think it's going to happen, I don't see it changing which is a big big shame."