In my opinion, the real impetus for opposing denationalization is enshrined in the (former) Clause 4 of the Labour Party, which implicitly lays down that the holding of the heights of the economy in common ownership is an necessary first step towards building a democracy. Any attempt to introduce democratic reform without this 'first step' will be fairly tame - society will remain dominated by the wealthy
The political situation in this country appears to have gone full circle within one lifetime. In the fifties we had a tranche of three Old Etonian Prime Ministers in a row. The change of government in 1964 appeared at the time to indicate a permanent new direction (I don't mean in reality, I mean in the way it was perceived when we first learnt about it when young).
Denationalization is all part and parcel of this movement back to rule by the wealthy - Old Etonians and all. It suits the economic interests of the wealthy and it further reduces any influence from the rest of the population.
As stated famously by Lenin but probably in reality probably equally attributable to any numer of people: "People who carry out revolutions by halves, dig their own graves". If you just carry out piecemeal nationalization, then it is no surprise at all that if the boot shifts on to the other foot (by, for example, the debiliating effects of mass unemployment for the last forty years plus) then the wealthy will not just restrict themselves to regaineing what had been nationalized but will go much further and grab as much as they can.
Effectively all the means of giving oxygen to ideas are in the hands of the wealthy and events proceed thus : the said mass media put out the idea that nationalized industries are inefficient and/or lose money, and a large propertion of their readers just regurgitate this parrot fashion, with no forethought, no undertsanding and no evidence.
This does lead to contradictions. George Orwell's book 1984 is based solidly on what he calls 'double-think', the ability to have two contradictory viewpoints and agree with both of them simultaneously. And the most obvious example of double-think is the idea that nationalized industries are inefficient but nationalized industries like the armed forces and the police are so totally super-efficient that no-one should even dare criticise them. Contrary to what some people think, 1984 is an attack on capitalism.
Historically the first major privatization occured in Deutschland during the 1930s. Information is (possibly deliberately) scarce. This is some information on this link - if you have more information I would love to hear from you.