Children gain clear advantages from grammar schooling. Often, to gain these advantages, they have had paid tutoring, tutoring from their own parents, or live in an expensive house near the school. Or all 3. None of this is any different from a private school parent buying advantages. So I don't see why well off parents using these schools shouldn't be taxed, if private school parents are to be taxed. Labour have been very clear that they do not agree with some children having educational advantages over others, particularly when their sharp elbowed parents have facilitated this.
The order to phase out grammars was made by the Labour government in 1965. This is from the BBC:
During the 1950s and 1960s, it was said, mainly by Labour politicians and egalitarian educationalists, that the selective education system reinforced class division and middle-class privilege.
In 1965, the government ordered local education authorities to start phasing out grammar schools and secondary moderns, and replace them with a comprehensive system.
The quickest changes were made in Labour-controlled areas, while strongly Conservative counties moved slowly or not at all.
A handful of counties and local authorities in England have kept largely selective schools systems, including Kent, Medway, Buckinghamshire and Lincolnshire, while others such as Gloucestershire, Trafford and Slough have a mix.
In other places, a few grammar schools survived in areas that were otherwise fully comprehensive, such as Birmingham, Bournemouth and some London boroughs.
In 1998, Labour's School Standards and Framework Act forbade the establishment of any new all-selective schools.