There is not cloudy legality in pre emptive striking if its not addresed in law, if the criteria of threat are met you may act before the threat does. (obstructing your path is a physical threat and they have initated a fight with you, plenty of people forget that and stuff of that nature are fights) If its a clear you cant do it, then you cant, if its a clear you can do it with these crtieria then you follow these criteria, but i dont think many places address it at all beyond common law. So it just falls under the same assesment criteria as using force to defend yourself, you can just act before the threat does. (as you should be able to as the first action usually wins, its one of those myths its unalwful when its really not in places) There is a reason why every good self defence cirrculem includes the fence and hitting the person before they hit you. Well, looking for and indentifying the threat, then taking follow up action.
And for the direct above, a flincing response is usually sufficent to break contact and a secondary point to self defence is buying as much time as possible and fighting back increases your odds of surviving a lot more than not. You make yourself not look like a victim so they will go to a easier target, and if they spend 30 mintues grappling with you in a public area there is a chance someone will along or someone will hear the scuffle.
Just not installing a nature of passivity/submission and defeatism could be argued as a decent self defence "training" as you would desuade anyone looking for a easy target once they have to fight you for 30 minutes.
If you cant go anywhere i would just echo what i read in dead or alive (paraphrase) "Just learn how to do one technique at a time starting with your strong hand, even if you can only straight punch well with your strong hand, it beats not being able to do anything and gives you a technique to use." (actually Geoff Thompsons Dead or Alive is a pretty good book if you are intrested in self defence)
I think thats a apt paraphrase i cant remember the wording for the life of me and cant fish the book out currently. I dont think punching is the hardest thing to learn by yourself if you get some good information on how to do it, the main issue is the latter.