The composition and nature of gang culture has shifted: gang members are getting younger; geographical territory is transcending drug territory as the key organising unit; and violence is increasingly important as an articulation of status, making it is increasingly chaotic. Secondly, gangs are mostly commonly found in urban areas of high deprivation, crime and violence, high unemployment and high family breakdown. Thirdly, gang members are predominantly male, in their teens or early twenties, from fatherless families and have had a negative experience of school, often having been excluded. Having identified the what and why, the report then sets out a blueprint for tackling gangs.
 · There are around 170 street gangs in both London and Strathclyde.
 · Gang members are 2½ times more likely to commit a violent offence than non-members.
 http://www.centreforsocialjustice.org.uk/default.asp?pageRef=314


I found even a whole online book based on gangs in England with its structure explanations and reasoning. Book's link is here.