One could say that punk can be seen as yet another event in a continuum of cultural resistance of which we find traces in the folk music of all past and living cultures, but that way of course there is no starting point that can be remembered. In itself, that would be okay, but people often tend to catalogue what they perceive in their environment. That is also the reason why there are so many genres and subgenres. Everyone catalogues everything because the amount of information is too chaotic for our limited brain. We cannot deny the reality. At one point in time, a consensus emerged to call a certain movement punk, and so this starting point arose.
Discharge actually exists since 1977. They are also part of that first generation, although they chose for a radically different sound later on. I do not see them as outsiders, but as a logical consequence of the evolution that had already been initiated and from which they took the logical next step.