As someone who lived through the 1970s and the punk movement it has to be said the punk movement as a whole was a reaction to the state of the country.
Britain was in a right mess at the time, with lots of strikes, civil unrest, and so on. In the late 1970s we had the "winter of discontent" (look it up on the internet) where society seemed to be collapsing.
While it was a protest, many of the people involved did not really have a clear message, they just wanted to protest. The Sex Pistols seemed to just want to upset as many people as they could and get as much publicity as they could (an early form of reality TV I suppose).
Someone like Johnny Rotten was not very bright and I dont think he ever knew what he was protesting about (he still doesn't). He just wanted to protest.
Also there was little youth culture on TV (no MTV etc) and all the media seemed to be run be older people so this was a way for young people to get a voice and get on to TV.
Musically it was a protest about some of the overblown rock groups who played long pretentious songs at the time (Pink Floyd, Yes, Emerson Lake and Palmer etc). Punk was simple and easy to perform, anyone could be in a punk band.
It was giving music back to "ordinairy" people.