If you're looking for a daily paper well to the left of The Guardian, can I interest you in the Morning Star? Available from your local newsagent, owned by a co-operative closely linked to the Communist Party of Great Britain. No need to go underground!
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 Mirror could be described as further left than the Guardian lately.
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.
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.