Martin is refused early release because he refuses to show remorse and doesn't say sorry.

That seems reasonable to me. I don't know a lot (anything, really) about the British parole system, but, to me, the key word is ``early''. If he doesn't have any qualms about having shot the guy, then why should he get out early. I don't know what the official stance on incarceration is in the UK (or if there is one), but it seems that at least one of its functions should be to try to prevent the crime from happening again. If he has no interest in avoiding it, then why should he be let out early?

Somehow I imagine that the burglar started running away as soon as he heard that shotgun being pumped and that there was no real need to shoot the guy, but that's starting to question the sentence, which, as you say, isn't the issue.