You are allowed to stab an intruder who breaks into your house. Three cases in as many years. The only reason the cases went to court was to determine if the intruders were invited or not, even the Prime Minister commented on the cases saying all MPs from all parties agreed with the courts to pursue no prosecution of the three homeowners... The intruders all died of their stab injuries.
In law - and a good law, I think it is - you are only allowed to use "reasonable force" when confronting an intruder (or indeed, when protecting yourself in, for example, a street mugging). For example, lets assume that some 18 stone, super-fit gym freak, is awoken by a noise in the night and rushes downstairs to find some terrified, weedy burglar trying desperately to escape through the patio doors, the sensible option would be to grab, restrain, and tie up the thief. However, if gym freak man, in a rage, punches the intruder senseless and then stabs him 12 times through the chest whilst he's lying on the floor, he would, rightly, in my opinion, be charged with murder. An extreme example, perhaps, but it does illustrate why the principle of "reasonable force" is necessary.
But with a high proportion of burglars now carrying weapons it means the use of a knife by the house owner is now de facto reasonable force. 10 or 20 years ago it was different because most burglars only carried the tools of their disgraceful trade, these days they are prepared to use force... even to the extent of burglary in broad daylight with the intent of producing their knife to carry out their thievery knowing full well the owner is going open the front door when they knock. Reasonable force is at play as you point out, but reasonable force is relative to the statistics of the crimes at the time.
I take your point, Project - but I'd be interested to know whether violent, aggravated burglaries by criminals are really on the increase or not. I seriously suspect that most lone burglars do not want a confrontation at all, and would flee if confronted. The right wing press, and tv programmes such as Crimewatch, heavily feature sickening and violent burglaries because they're "sexier" and more interesting to read/hear about - but they give a skewed perception to the public of the true picture. If, God forbid, I were faced by a knife-wielding burglar in my living room, I'd have no hesitation in deploying a similar weapon myself - but I still reckon that 90% or more of burglars are of the non-violent sort.