Whilst that is indeed true for the population in general, politicians are a bunch of people self-selected on being the kind who wants power.
That kind of personality is generally less trustworthy (and more on the sociopath side of the spectrum) than the general population.
There's actually a study published ages ago in the Harvard Business review about corporate CEOs (so, not politicians but in many ways similar) which found that the ones who got the job not because they sought it but because of other reasons (for example, the CEO died and they were the next in line) actually performed better (as measured by the performance of the companies they led compared to the rest of their industry) than CEOs who had sought that position and, even more interestingly, the most self-celebrating showoff CEOs were the worst performing of all (from my own participation with politics I would say those would be the closest in personality to top politicians).
Further, there are various pretty old sayings (back from the time of the Ancient Greeks and the Romans) about the best person to get a leadership position being the one who doesn't want a leadership position.
So I would say that most politicians in parties with higher chances of getting power (so, in most countries, the two largest parties) are crooked (not specifically corruption - such as getting money to pass certain laws of using certain companies for government contracts - but more generally using power, privileged information, influence and connections to benefit themselves even to the detriment of those who voted for them: a good example of crookedness but not corruption is how some US Congressmen use insider information they get in some Congressional Committees to profit in stock market trading).
The Guardian absolutely is capitalist (neoliberal, even). Just go check back on their campaign against Corbyn (a leftwinger who won the Labour Party leadership from the New Labpour neoliberals some years ago) which included such memorable pieces of slander like calling a Jewish Holocaust Survivor an anti-semite because of him in a conference about Palestine comparing some of the actions of the government of Israel with those of the Nazis, this done in order to slander Corbyn by association since he was in the same panel in that conference.
Also you can merely go back a few months to see how The Guardian supported Israel well into their Genocide (though they seem to have stop doing it quite as eagerly in the last few months).
Last but not least they very openly support in British elections the Liberal Democrats (who are neoliberals) and the New Labour faction of the Labour Party (also neoliberals) and very often have pro-privatisation articles on UK subjects and are never for bringing things back into public ownership even when privatisation has failed miserable to give better services or lower prices.
I lived in Britain for over a decade and read The Guardian for most of it, so maybe The Guardian's political slant is clearer for those familiar with British Politics.
I do agree on The Intercept and Democracy Now! though.
Can't really speak for the others with any knowledge.