aog,the reason you said " gangs are mainly made up of young people, mainly in certain neighbourhoods, and of black youths" was to contradict the speaker's statement that the problem was not pertinent to age group, neighbourhood or ethnic background. Now you say that the reason you said that was because the conference was in London and you were seeing it from a London perspective.
It was called "Tackling Britain's Gang Culture". Next time a conference on Britain is held in London, will you read that as being about London too, rather than Britain, aog?
Has it occurred to you that the number of black gangs may be proportionate to the number of young black people living in an area? Therefore that there are more black than white ones is only to be expected? (Have a look at the map)
And why do you think arguing from a London perspective and citing London makes the argument valid for Britain?