Any evidence to support that claim, by any chance?
Most reputable private landlords want regular payers and an easy life.
They dont want trouble, they dont want their very valuable property damaged.
Therefore most of them will bend over backwards to vet any potential tenant with as fine a tooth comb as possible.
The Housing Associations, whilst not as finiky as the private landlords have exactly the same issues and problems
Therefore this leaves those potential tenants who are a 'problem' with the local councils, who in many case really do not seem to care about the people or the quality of the housing.
Anything (just) is better than a cardboard box on the street.
As they do not keep an eye on their own housing stock you end up with sub letting as a fairly common problem (almost unheard of in the private sector where regular property inspections take place, and fairly rare in the housing associations, although some associations are better than others)
Of course there are exceptions in all sectors, the far west of London 'beds in sheds' issue is one example and the Bristol HMO's is another, but if you want to see rife sub letting in appaling conditions then look at some of your local council owned properties.
If you are in the coutry illegially, therefore working in the black economy, you can't rent a house via the private sector or housing associations, so you turn to the council or council sub lets, it's the only place to go.
Having said which, this is a country in the top 10 most rich nations on earth. Everyone in this country should have safe, warm and dry bedroom, a decent bathroom and somewhere to cook and live.
I could cure the current situation in about 15 years without building on the greenbelts, but it would take the loans of money and commitment of the 1950's when we were building 250,000 houses a year. The base problem is Maggie got the idea of selling the council house right, however the fundamental problem is that for every house sold they should have been building between 1.5 and 2 new homes for each one sold. We now have 25 years of catching up to do.
Bottom line is over a 30-40 year period it would not cost the country a single penny.