You have to look at it in the context of the UK broadsheet market though. On the right, the Times (usually) and the Telegraph and if you'd consider it a broadsheet, the Daily Mail too.
The Guardian is the only consistently left-leaning quality paper at least in England and I don't blame them for sticking to their guns.
The Guardian is bourgeois establishment, not left-leaning. It's been consistently hostile to genuine left-leaning views for at least a decade - probably longer. Its editorial policy reliably paints anyone to the left of Tony Blair as a lunatic extremist.
Unfortunately this includes most paid-up members of the Labour party, whose perfectly legitimate views it reliably tries to marginalise.
And it runs campaigns like this one, which seems to be part of an attempt to conflate honest debate and criticism with the worst excesses of abusive online trolling.
It's true it's marginally to the left of the printed mainstream, but that only shows how far the Overton Window has been pushed away from moderate centrism in British politics.
The Guardian is the only consistently left-leaning quality paper at least in England and I don't blame them for sticking to their guns.