the paper’s ostensibly liberal/progressive line

They're aligned with the Liberal party, which is a centrist party which is seldom if ever progressive. The Guardian does put up some articles by progressives, on occasion, but they also publish articles by conservatives. When the Labour Party was led by Corbyn, the Guardian was consistently critical of Labour policy and bought into the rightwing press's phony accusations that Corbyn was antisemitic. Overall, the Guardian's core politics are those of the metropolitan bourgeoisie, as can also be seen by their lifestyle and media commentary, as well as their general smugness. And on economic matters, their coverage is utterly useless. On that, the Economist and the FT are far superior, despite their occasionally odious politics in their editorial pages.

I still read the Graun, though, since the rest of the British press is far, far worse.