CJ112 Criminal justice cannot be successfully imposed from above, but needs as far as possible to be a product of a living, democratic community. The basic institutions of Green justice should be community-based and relatively informal in nature. They should provide maximum potential for public participation.
http://policy.greenparty.org.uk/mfss/crime.html
Just going through some of their policy documents, while their energy and transport policies look reasonable, their economic and military policies strike me as largely unrealistic.
(Update: Actually I should mention that I do disagree with a large section of the Energy document; I can't see any realistic way to drastically reduce CO2 emissions while simultaneously ceasing to use nuclear power. However, I think both are desirable goals.)