The UK's CPS guidelines.

http://www.cps.gov.uk/legal/s_to_u/self_defence/#Reasonable_Force


Since the availability of a gun (as in Zimmerman's case) would have been unlawful in the UK in and of itself, I do not see that Zimmerman's use of the gun could have been reasonable.

Comparably the UK coroner has found this shooting to be an unlawful death -
http://www.bbc.co.uk/news/uk-england-london-23193571

We await hearing whether the police marksman involved is to be prosecuted. The deceased was in a car with at least three guns, was a career criminal, but the marksman's evidence that he thought the deceased was trying to shoot at the marksman and had failed to go down after being shot 6 time, when the marksman delivered a classic "double tap" headshot was rejected by the coroner. Because it was an inquest not a prosecution it has come out that the marksman has killed twice before - which may make a fair trial by UK standards hard to achieve.