In the uk you can actually be arrested for defending your house, even if you're in fear for your life
Will gramps get jail time like countless others before him?
In the UK it depends on the circumstances - it's all related to what's regarded as "reasonable force".
Lets say a burglar tries robbing your house - you beat the shit out of him, fine. Then you call the cops while he's apprehended and they'll arrest him, not you. However, if you render the burglar incapacitated and then kick him in the head for the hell of it then while waiting for the cops to arrive then you'll be arrested because that's classed as "unreasonable force".
Basically at the moment in the UK the CPS are going to be wary about prosecuting a homeowner defending their own property if it can be shown they used reasonable force because of public sentiment.