If you define "dangerous" as "lost likely to get stabbed or beaten up", then anywhere. It's no more dangerous than if you were to mouth someone off at a bar in any other city.
However, gangs do operate in suburbs. I'm from West London and I can tell you that Hayes is a pretty shady place. 12 people (of a non British ethnicity) kicked down my friends flat door, held a knife to her throat, stabbed her boyfriend and stole his wallet.
All that for a wallet. They had to pay for the damage, too.
Just be wary.