It doesn't say, which is a little weird. This article explains it better.
Basically as I understand it, before the 1980s, the government owned a lot of the housing and rented it to people at fixed prices. This meant that renting your property out purely for profit was tough, and a lot of landlords actually sold their property to the government as council housing. That changed under Thatcher, who enabled private sales of the council housing, which originally sounded like a good idea (you can own the home you're already living in instead of renting it from the government), but increased privatization led to rent for profit led to inflation of monthly rent led to oh no.
The simple fix I suspect, is for the government to start buying up properties again for rent-at-reasonable-prices to tenants, competing with private landlords and poking a hole in the bubble of ever-increasing rents (with popping the bubble giving a lot of extra leverage of societal benefit as compared with the amount of money they're actually putting into the system.)