IMO the underlying problem here isn't the definition of reasonable force. It's the lack of a cap on the punishment of a householder going over the top.
It is possible that you have situations where the only difference is that one householder stabs the would be burgler one more time than the other and gets a mandatory life sentence whilst the other gets no punishment at all. Both murder and manslaughter carry a mandatory life sentence, although eligibility for parole and whether to suspend the sentence are up to the judge.
The punishment needs to be capped at something like 2 years in prison for someone who commits murder on a would be burgler and the sentencing guidelines need to make it clear that is only ever to be used for a repeat offender where they have deliberately planned the murder.
Pretty much everything else should be at worst a couple of months and it should be suspended. Going through court for this is enough punishment and deterrant for someone that essentially went over the top defending their house. Also, the burden of proof needs to move when someone's house is invaded. It should be up to the prosecution to prove beyond reasonable doubt that the house wasn't being burgled/robbed to pursue a householder fro murder.
Currently, I believe that although the CPS have to prove murder beyond reasonable doubt, to use the "self defence" defence the accused does not simply have to introduce reasonable doubt. IIRC they have to show "on the balance of probabilities" or somesuch that it was self defence.
Secondly, this piece of crap government should start funding charities for victims. There are over 30 government institutions to assist the offender and not a single one for victims yet they have the balls to add a victim surcharge of speeding fines.
A properly set up charity should then assist the victims in civil recovery from the scum. Yes, you can currently sue any burgler that got caught and everyone in that situation should do so and should have the free assistance in doing so.