The relevant UK law (as it seems to me) is discussed in part 4 of the Law Commission Report here -

http://lawcommission.justice.gov.uk/docs/lc290_Partial_Defences_to_Murder.pdf

My first thought is that it would be impossible under UK law for Zimmerman's use of a firearm to have been "reasonable force" for the purposes of the law of self defence since it would have been impossible under UK law for Zimmerman's carriage and discharge of a firearm to ahve been lawful.

My second thought is that if one ignores the firearm element the defence of self defence would only have been possible if Martin attacked Zimmerman rather than the other way round. This becomes somewhat convoluted in that (assuming that Martin did not intend to kill Zimmerman - possibly a large assumption) if Martin genuinely and reasonably apprehended an assault on him by Zimmerman (a belief that I would have thought eminently reasonable) he would have been entitled to use reasonable force to defend himself from that apprehended assault, and the known damage to Zimmerman (if caused by Martin, another possible issue) would seem to be within that level of force. But (and so it goes round) it seems inescapable that if Martin attacked Zimmerman ( believing it to be in self defence) it is hard to doubt that Zimmerman would be likely to believe that he was being assaulted - in which case he would be entitled himself to use reasonable force.

So there are two pathways to Zimmerman being convicted of murder. First if it be found that he attacked Martin, I think he would be guilty of murder under English law. Second if it be found that he was attacked by Martin but then used excessive force, he would be guilty. I do not see how, under English law, the use of an unlawful firearm could be anything other than excessive force.

Another issue of fact would be whether Martin was merely trying to get away when Zimmerman shot him. If so the English law is beyond doubt, Zimmerman would be guilty.