First, the idea of chronology was removed; themes replaced periods and children were encouraged not to learn about great figures from the past but to “empathise” with common folk by imagining they were a peasant in Mediaeval England or a chambermaid in Victorian London. Then the curriculum was narrowed by teaching the Tudors and the Hitler period over and over again. Finally, history was made non-compulsory after the age of 14.
Why did all this happen? Because the entire British political class became committed to multiculturalism and the putative wonders of diversity, a state of affairs they had engineered over half a century from 1945 by allowing mass immigration. To enforce multiculturalism it was necessary to destroy historical understanding and national sentiment amongst the native population, because the success of multiculturalism relied on the native population being denied not only a public voice but the intellectual and emotional tools to oppose the imposition of multiculturalism.
The other side of the multicultural coin was to incessantly promote the various cultures of immigrants as things of great value which both excited and strengthened the naturally separarist tendencies of the and indoctrinated the native population with the idea that these people have culture, you have none.