These things go in cycles. Back in the bad old 1970s, people were tired of "being held to ransom by the public sector unions" (as they saw it; although there was a degree of truth to it) . My Dad worked for the council roads department and he used to complain how long it took anything to get done, except on Sundays when they used private contractors. So there was the obvious attraction that private companies do tend to be a bit more efficient, at least in terms of not allowing dead wood to hang around. And to be fair anyone who works for a public sector organisation even today can tell you about colleagues who are either never there because they're "off with stress" or who do bugger all when they are in the office.
So Mrs Thatcher, daughter of a small business owner, decided — after reading some articles by various right-of-centre economists — that the private sector should do everything. Yes, the private sector would expect a 15% margin, but since they would in theory be 50% more efficient, that's win-win, 35% saved (*) for the taxpayer, better and quicker services for the users.
Also, her marketing people came up with the brilliant idea of getting the public to buy the shares. Older readers will remember the slogan "Tell Sid" for the British Gas (I think it was) privatisation. It was going to be one huge share-owning democracy with all of the directors answerable to Sid (and Doris).
Unfortunately, capitalism is a bit smarter than that. So the various privatised companies went from being owned by Sid to being owned by hedge funds, and giving worse and worse service while paying more and more to their shareholders. But because they were mostly local monopolies, unlike Alfred Roberts's grocer's shop, the customers couldn't go anywhere. In effect, instead of public sector unions holding people to ransom for a 15% pay rise, now you had a legally mandated transfer of the taxpayer's money to smiling men (and a few women) in suits, who told everyone how "entrepreneurial" they were while being nothing more than rent-seekers.
This has led to a problem that is known in economic theory as "moral hazard" (basically, "pay me more money to provide social care or this old lady will die of neglect, sorry about that but business is business"), and despite the fact that is was inevitable given that the government did not structure the privatisations to avoid it, it was not sufficiently taken into account by the people who imagined that they were turning the UK into a nation of little businesspeople.