http://www.lawcom.gov.uk/docs/lc290(1).pdf
This is a Law Commission report on defences to murder. It spells out very clearly the English rules. You will want to look at from page 72.
The short read version is: http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2005/02/02/nburg202.xml
The Law Commission document stresses, deep in the text, that at a trial, if the defendant says 'self-defence', 'prevention of crime', the defendant is creating a reasonable doubt. It is then up to the prosecution to destroy this doubt. That is, to show beyond reasonable doubt that the claim is untrue.
This evidential rule is why people get off. A killing + self-defence / prevention of crime = no crime.
Now apply this to Stockwell Tube.