It's probably best put by the first sentence: the mainstream left of British political opinion - which turns out to be almost wholly center-left, even Corbyn is a social democrat.
More seriously, The Guardian is a (social) liberal newspaper. It's politics are generally those of the 1980s Social Democratic Party, not the left of the Labour party. It's biases are metropolitan, if anything.
That there is no quality paper to their left is mainly because the British newspaper market is already crowded so it's commercially difficult. It doesn't mean that The Guardian can be reasonably labelled as extreme.
The past few years especially has seen a shift in the Guardian's position well to the right to the point where even calling it centrist is probably not correct. This is likely a consequence of falling sales and reliance on advertisers - you'll see a large number of sponsored articles on it these days for example.