UK based web/ software development co-ops:

WebArch
http://www.webarchitects.co.uk/

Software.coop
http://www.software.coop/

Co-operative Web
http://web.coop/

Wave
http://www.wave.coop/

The Creative Co-op
http://creative.coop/

Alpha Communication
http://alpha.coop/

Atomised
http://www.atomised.coop/

graphics.coop
http://www.graphics.coop/

London Developer Co-op
http://londondevelopercoop.com/

Nomad
http://nomad.coop/

Agile Collective
http://agilecollective.com/

3 Mules
http://3mules.coop/

Grow Collective - do they still exist?
http://gr0w.com/ re-directs to http://2008.gr0w.com/ and latest post seems to be from 2008 :-/

Cooportunity
http://coopportunity.org.uk/

KABA PC
http://www.kaba-pc.org.uk/