Someone has just tweeted in a very timely fashion a link to this 2010 report on Gangs, which was commissioned by Labour and why they say hasn’t been acted upon by the current Government. It includes some definitions:
How the police define and understand gangs
There was not one accepted definition of a ‘gang’ in the police service and it was clear from various personnel in the same police areas that definitions varied locally. For example, police officers gave us definitions varying from just a group of young people gathering on streetcorners to organised crime networks. However, the most common understanding of a ‘gang’ held by police intelligence units was a group of people who identified as a group, usually through a name, who were geographically based and had some involvement in criminal activity.