Robbery depends on where you are. And how vulnerable you look.
Turning into a fight is your discretion really, if you hand over your shit they'll more than likely leave you, although I get the impression you're not that type of guy.
As for self defence... the term thrown around a lot is 'reasonable force'. This means if someone tries to mug you without using or convincing you they have a weapon, giving them a good slot on the nose is fair but breaking their legs or killing them is over the top. If they're telling you they have, a knife or gun, it's different, and if they genuinely look like they'll kill you, killing someone is kind of acceptable. You can still get a criminal sentence for it, it really depends on the situation and the jury in the case.