Some notes I've consolidated from a few places with regards to actual self-defence. I've covered a targeted assault rather than a 'sucker-punch' attack as generally it's incredibly difficult to defend from a an attack like that (from behind? no announcement? while your attention is naturally elsewhere/not suspecting of an attack? Not going to be able to diffuse that kind of attack and would be far more difficult to engage fully)
If you are targeted for a fight (i.e. not sucker-punched):
Ego - either threatened or given a chance to cheaply grow - is a massive aggressive drive. Instead start off meek. There's no boasting, no "man-points" for fighting someone who doesn't appear to be a threat.
What if nothing has worked... at this point you *still* do not brag that you've trained in any martial arts. Again this just creates a "man-points" situation for the attackers, if they beat you they have bragging material, so they have another 'reason' to try and win.
Anyone, or any school that tells you that your self-defence aim should be to incapacitate your attacker(s) over all else is wrong. Morally and legally (UK law anyway). Your goal should always be to get yourself into a safe environment, whether by giving them possessions, fighting, running, or some mix.
By UK law you are not allowed to escalate a violent situation. That generally means if the aggressor stops being a threat that you must stop whatever aggressive measures you were taking to defend yourself. You also cannot 'increase' the weapon type being used, regardless of difference in size or numbers in their group (empty hand < sticks < knives < guns etc.).
Also keep in mind it really does not take much to accidentally *kill* someone (think about both you and your attackers). Them, or you, falling to a hard surface and hitting their head can be all it takes. There are a number of deaths that have made national news that have been due to *one* drunken punch, and the victim hitting their head badly on the pavement.