England has a long history producing artwork against some institution, only for that institution to get worse over time. George Orwell wrote about the dangers of authoritarianism and surveillance, and since then the UK government has only ratcheted up their surveillance and authority. They also made a movie called This is England which straightforwardly depicts young English nationalists ruining their lives with nationalism, and 20 years later there are more nationalists in England than at any point after WW2.

Will Banksy's legacy be more or less the same?

England has gotten more liberal over time, not less. I'm not following your logic here. It seems you're wanting to criticize the government of the UK for being authoritarian and ratcheting up the surveillance state, but simultaneously criticize nationalists and link them to this government, but nationalists and right-leaning groups haven't really been in charge of the UK.

"They also made a movie called This is England which straightforwardly depicts young English nationalists"

Not sure who you think "they" are but "This is England" is superb. It deals with a lot of issues, way beyond just nationalism and the like.

Perhaps you would like to fix your gimlet gaze on "A Clockwork Orange" and deliver a further withering critique.

A simple explanation regarding the increase of the number of nationalists within England is the population has increased. QED.

almost as if "England" is more than one person!

This is such an odd comment. People in arts and letters warning about some element of society or culture and then that element growing in strength is something that can be found in most countries, and doesn't seem more prevalent in England than elsewhere.