The word that sprang to mind was “keening”. I was walking towards Green Park late on a chilly night, yards from the glowing lights of the Ritz, when I heard it. I can’t remember ever hearing a human make that kind of noise before.

As I reached the Tube station, I found the source – a man sitting with a woman on the pavement, both of them swathed in blankets. People hurried past, unsure how to react to this guttural, animal sound.

What is the right thing to do when confronted with a person who is street homeless? One of the most popular articles ever published on the New Statesman website argued that “you should give your cash directly and unconditionally to homeless people”. Its author, Matt Broomfield, pointed out the many structural barriers to receiving help, such as hostels that require a referral, or services that risk an encounter with immigration enforcement. “Many street beggars are addicts, yes,” Broomfield added. “Do addicts not deserve food?”

Even homelessness charities struggle with advising us on the best response, given the narratives that swirl around the problem. In September, Crisis expressed its discomfort with the rhetoric used by “targeted giving” schemes, which encourage a charity donation instead of sticking a few quid in someone’s cup. “The current focus on begging puts all the blame on the individual rather than on the real issues – the lack of affordable homes, access to health care and support services,” said John Puzey, the director of Shelter Cymru. “We encourage the public to use common sense when deciding whether to give money to people who are begging. The truth is you have no idea how hard their life may be. If you decide to give money, make it a gift with no strings from one citizen to another.”

That’s what I did at Green Park, handing over a fiver and asking the couple if they wanted help finding a hostel. “We want to get a B&B,” the woman told me. “I was raped at a hostel.” I had nothing to offer but sympathy and turned into the station. Four stops away on the Jubilee Line, at London Bridge, there were more homeless people under the arches between the Tube and the overground. One, an old man, sat in mute misery, staring at the floor. I didn’t have any change left. “We’ve got to get the Tories out,” said my husband.

Street homelessness is only the most visible symptom of a larger crisis. The charity Shelter estimates there are 320,000 people in Britain sleeping rough or in temporary accommodation, plus thousands more “hidden homeless” staying on friends’ sofas, or sleeping in cars and sheds. London is the homelessness capital of Britain, with one in every 52 people lacking a secure home.

The recent rise in rough sleeping is now obvious: there are tents in the church portico in front of the BBC’s headquarters in Portland Place, so every politician who goes on the Andrew Marr Show, or Today, or the news channel, will pass by them. There are people sprawled on the pavement of the Strand, dozing in the daytime, hideously vulnerable to being tripped over by commuters rushing to work. The capital’s Sikh community distributes free food on the streets from vans, as part of the “langar” tradition, where all visitors to a gurdwara are offered a free vegetarian meal.

There are more tents in my local park in Lewisham. I expect there are some in the shadow of the half-empty buildings of luxury flats dotted through London, waiting for an oligarch’s feet ever to touch their carpets. In a grim irony, one of the capital’s most well-known homelessness charities is called Centrepoint. The nearby building that inspired its name, close to Tottenham Court Road, has now been converted into 82 luxury flats, with a £55m penthouse. In October, Mike Hussey, chief executive of the developer Almacantar, announced that his company was halting sales of the 40-odd remaining units in the tower. “We see no point in chasing a market that is increasingly detached from reality,” he said. What he meant was, incredibly, that the offers he was receiving were too low – for some reason, people were not prepared to pay £1.8m for a one-bedroom apartment – and he wasn’t prepared to slash the prices.

The homelessness crisis is the result of a “perfect storm of spiralling rents, welfare cuts and a total lack of social housing”, according to Polly Neate, chief executive of Shelter. The government acknowledges the problem, and has committed £100m to tackling it. In the spring it imposed new duties on English councils to prevent homelessness – although local authorities are already struggling with reduced budgets, so increasing their statutory duties but not their cash flow means every new B&B referral is money out of a library or leisure centre.

The stupid design of Universal Credit, which swaps housing benefit paid directly to landlords with a lump sum given to claimants, has exacerbated the situation, as has the fact the new benefit is paid in arrears. The lack of social housing is not a new problem (and, of course, neither is homelessness; I remember my quietly Christian parents serving “hostel suppers” in the 1990s). But it is now acute. In 2016/17, just 1 per cent of house-building was completed by local authorities in England and Wales, compared with 7 per cent in Scotland.

Because I’m not a fan of Jeremy Corbyn, I’m used to being accused of being a Blairite/Tory/centrist dad in disguise. But nothing makes me long for a tumbril like the London property market. Forget the policy levers for a moment, and just focus on the morality. Don’t you feel… disgust? A city where luxury flats stand empty and desperate people shiver under railway arches.

No wonder we walk on by, telling ourselves that the homeless deserve it, through drugs or drink or bad decisions. How else would we live with ourselves?