From the Crown Prosecution Service, '
Householders and the use of force against intruders':
'Anyone can use reasonable force to protect themselves or others, or to carry out an arrest or to prevent crime.
You are not expected to make fine judgments over the level of force you use in the heat of the moment.'
Basically if (God forbid) it happens to you and if you somehow ended up having to explain yourself, your lawyer will be able to argue in your defence that you did whatever you deemed necessary in the heat of the moment. The scumbag who tried it on (again, God forbid) would likely plead guilty upon arrest, even if he had been hurt, and so you'd probably not have to give evidence in court or even get a lawyer.
Based on what you said about being 8 stone, diabetic etc., the CPS and the police would all agree that you were acting in self-defence. The use of insulin might raise an eyebrow or two but read the second sentence quoted and have a look at the article itself to get the full extent of the law on this. If in some kind of ridiculous miscarriage of justice you found yourself having to get a lawyer to explain your actions for you, they would be able to do so and you would not be charged with any crime because you haven't committed one.
And again, burglars who are caught - whether by being fucked up during the action or by being caught later on - will tend to plead guilty due to the evidence and sentencing (judges/magistrates go easier on you if you plead guilty). If they are professional burglars, they are highly unlikely to try and burgle a home when people are in it but if they do, don't try to stab them with insulin unless you are sure that you can disable them as they usually work in teams, at least pairs, and they usually don't want to hurt you - they just want your stuff and we are talking about pros who target properties with high value goods in them such as jewellery. Again, these people are more likely to break into a place when they know that its empty.
What you're talking about is rag-tag hoodlums chancing it and these aren't pros, they are stupid but potentially highly dangerous so be careful, but feel free to fuck 'em up if you can, as much as you can, "in the heat of the moment", not quite making "fine judgements" over the level of "reasonable force"

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