I'm not a Brit and did look into the matter. I'm currently studying here though.
Self-defense being punished is a matter of bureaucracy.
Even in the case where a weapon was used for self-defense, none were charged for the act, but for the possesion in public. (pretty much everything is illegal, there's been an argument that Bill of Rights is the lawful authority in offensive weapons act to allow carry weapons for defence and when you are defending direct physical attack, it's necessary force, not reasonable force)
What IS true is that it is not easy as to get self-defence approved as you would in TX or FL.
I know a guy who carries a Monadnock baton daily, he's informed police and home office, but they can't do anything about it because of Bill of Rights. He actually took it to high court and proved his case that citizens still have right to carry anything that falls under offensive weapons act.
Tony Martin(the farmer) shot someone that was running away, at least according to autopsy and such, I don't think that would have got approved anywhere. but Tony DOES deserve sympathy.
But do not forget that he was originally charged for freakin murder. he ended up getting manslaughter charge.
I for the name, but there was a case where this guy shot this scumbag (I dunno he died or got injured) and got approved and the dirtbag didn't have a gun, but the dirtbag stole a shotgun from him not long ago and he had a good reason to believe that he had a gun.
He got away with no charge.