How do I deal with idiots and chavs that are hell bent on causing violence?

The other day me and a couple of mates were walking down the street minding our own business when all of sudden this 20 year old chav throws and muffin at my friend for no reason it was completely unprovoked and as it missed him he shouted back at them "nice shot" in a sarcastic way as it was a terrible shot. The chav and his rough **** mates turned round and shouted "oi prick" i thought they were talking to me so i replied "are you talking to me?" they said "no i want to talk to that fat ginger ****" it was at that point when they completely spun the story and implied that he said "ill smash your face in" when he simply rideculed at the fact that they were being complete tools in the first place . They began backing him into a corner and grabbed his drink at threw on the ground and rode off. my friend was almost in tears as he isnt use to that kind of stuff happening but even though it wasnt me in that situation he was looking to me for help and i just kept thinking i do not want to get involved maybe it was fear of getting brutally beaten or the fact i had an exam that day I do not know but that is irrelevant as my question is What do i do in that situation, every single person ive been to for advice just tells me to stay out of it and keep my head low but how can i do that when i see i friend in real danger and im just such a snob that i wont do anything. ive been in fights before but for some reason i was just powerless to do anyhting. what should i do in future if an event like this occurs again???? HELP