I agree that "not voting for the Tories" was pretty much the main driver, but these are not "new options".

The Brexit Party's "surprise" increase this year, was in many ways just returning to the 10-15% that they received as UKIP in 2015.

In the two elections in between, they agreed to not contest many of the Tory seats, to not risk "splitting the vote" to help keep "evil Jeremy" out of power.

The Tory vote + Brexit Party vote, added together, is lower than the number who voted for either Boris Johnson, Theresa May or 2017 Jeremy Corbyn. Fewer people voted for Keir Starmer than Jeremy Corbyn in 2017 or 2019 - so technically the biggest change in vote is probably "did not/unable to vote this year", with an increase of 3 million.

As ever, "didn't/unable to vote this year" won yet another successive landslide victory of about 20 million, or about the same as the top three parties added together.

Anyway, apologies for the tangent. The graph is particularly looking at younger people, who are on average more left leaning, and have become more so in the last 40 years. Though the recent mainstream politics/media shift towards the far right is absolutely terrifying, I don't think it's reflected in the young people shown in this graph.