Reading this Daily Mail article claiming that 64% of people walk on past a gang of teen troublemakers, I can only wonder - what's wrong with the remaining 36%? The poll claims that 27% overall would step in to stop a gang drinking and verbally abusing passers-by, and I'm sure that this overstates the numbers who would actually intervene - still, any rational evaluation of self-preservation would force the average passer-by to avert their eyes and keep walking. I do not, of course, include Chuck Norris or Jack Bauer in that category, but I understand they seldom visit South London or Birmingham
If drunken teens are abusing bystanders, they're not going to stop when someone they don't know from Adam tells them to stop. Why would they? They've been brought up in an environment where there are no effective sanctions on bad behaviour. Even if they've been in the criminal justice system due to thefts, assault etc. the presumption is to hand out suspended sentences to "encourage" them to go straight. Someone mouthing off at them is very unlikely to change their behaviour unless it's their mother - we assume that their father either isn't around, or is a waste of skin.
If it escalates into physical violence, the problems are worse. If you initiate the violence, even if defensively in fear of your safety, you will certainly be arrested and likely charged. The CPS might well not be able to make the charge stick, at least if you're sensible enough to keep your mouth shut and demand legal representation, but that's still several months of your life with the end of your professional career hanging over you. If they attack you without provocation, it's a toss-up as to whether a bystander will back your side of the story when the police turn up; they will claim you attacked them, and there's no downside for them to do this. Perjury prosecution? Don't make me laugh.
Practically, unless you get in fights on a regular basis, you are very unlikely to come out well from a 1 vs 2 engagement with teens; if they're drunk, their reflexes are slower but they feel less pain. Even 1 vs 1, you're gambling that they don't get a lucky hit or kick in and disable you. Once you're on the ground, they're going to kick you in the chest and head, even if they're a woman. A kick in the head in the right place can be permanently disabling, if not fatal.
What's the upside? You won't change anything by intervening. Drunk teens mouth off at bystanders every single day. Even if you scare them off today, they'll be back tomorrow. You can't apply any violent sanction, because you'll either be jailed (if disproportionate), arrested and charged (if proportionate defence), or in hospital or the morgue (if outnumbered or unlucky). Congratulations to the lawmakers and social scientists who have put this incentive scheme in place. May you be verbally abused and threatened daily.