Beyond the mainstream there's the broadly Communist Morning Star and the Trotskyist Socialist Worker, but they are very niche publications. Of the mainstream media probably the most popular newspaper of the Left is the tabloid Daily Mirror. More upmarket and centre left rather that out and out left is the Guardian. (Dubious.) Of political magazines there is the New Statesman, again centre left rather than out and out left - though compared to the very right political spectrum in the US they would be seen as left wing.
Blogger: Let's not forget The Independent!
Hard to agree with this assessment above. Although there is very occasional evidence of sanity in The Guardian - it is more a question of the bitterness which sits in the vast majority of articles published.
The Mirror lacks any balance whatsoever. It seemingly never examines all facets of any argument.