This has been driving me crazy for months! I hope someone remembers this.
It was probably but not definitely a YA book, though probably not called that at the time. It was Science Fiction. It was AT LEAST pre-1988, and read (and probably published) in the UK. It might have been from Penguin, not sure. It was not a very thick book, the paperback was maybe 1 centimeter thick.
The following details may have some mistakes - it's about 30 years since I read this.
It was set in a future Britain (not the UK, from what I recall).
The hero, a male, was either about to, or had just reached maturity, and was expected to start adulthood. He was part of the privileged elite who ran Britain an educated clique called the Ests or Ents (unless that name was for the revolutionaries), and his father was a significant member of that group/the government (perhaps even the leader), who expected great things from him (and obedience).
Instead he runs off and meets with the revolutionaries (who are losing), and perhaps falls in love with one, after overcoming their initial distrust. The revolutionaries were not part of the ruling group, though some may once have been. I think he tries to tell the revolutionaries that he can persuade his father to do something, perhaps go easy on them or make peace.
At some point the hero breaks into his parents' house (maybe through a skylight?), where I think he meets his father (unexpectedly sitting in a chair waiting for him?). I seem to recall he expects his mother to be there and wants to talk to her. I'm pretty sure the father tells him it's been a trick, they've been tracking him and he has betrayed the revolutionaries (and his mother is not in the house). His father expects him to rejoin the clique and forget all this nonsense. I think that at the end of that meeting guards move towards the house to catch him (and possibly any revolutionaries who came with him, I don't remember).
In the end the hero returns to some sort of camp or guard post, either under guard or to do something for the revolutionaries, and meets his father. I think the father pulls a pistol on the hero and threatens to shoot him, or maybe his revolutionary girlfriend, but somehow the hero ends up holding the pistol on his father, and I think he shoots him, to his father's surprise.
A few other details; I think the hero (and maybe the government) was based in Cambridge or Oxford. I think there was a wall or defensive perimeter around the city that the hero goes through, out into the wild countryside. I think the father's house was out in the countryside, though that seems unlikely, so maybe it's just in a wooded area within the perimeter.
Oh, and I think the cover had a futuristic car on it that looked a bit like the Lotus Esprit from James Bond (The Spy Who Loved Me).
I really hope someone can help identify that!