I looked up The Workers Music Association or WMA (est. 1936) mentioned by Fred. On their web site they have this to say about their origins:

"The story of the WMA
It began life in 1936 when five London Labour Choirs met to perform together at a time when the world was hurtling towards a struggle to contain the menace of fascism; embodied in the conflict of the Spanish Civil War; the development of the holocaust; wholesale genocide, and the suppression of human spirit.
Alan Bush ... with the support of many like-minded people founded the WMA as a response to those conditions and seeing the need for a body to be the spiritual home of Workers who could express their hopes and wishes through music, about the way the world should be."

And about their objectives:

"The WMA is dedicated to encourage the composition and performance of music with special regard to:-
Music which expresses the ideals and aims of mankind towards the improved organisation of society.
Music which exerts an influence against the social injustices of our present society.
Music which encourages and reflects the activities and aspirations of the labour and peace movements for a new society."

Furthermore they have a list of current "radical choirs" both formally affiliated to the WMA or otherwise less formally associated with them, and it seems there's quite a number of them out there too:

http://www.wmamusic.org.uk/Radical%20choirs.htm

A number of descriptors are used by these choirs, including "Community", "People's", "Clarion" (as referenced above), and of course "Red". As might be expected, barring London, they all appear to be in the North of England.