. . .Homeowners can legally use lethal force defensively in the UK. This has been tested in the courts time after time. What we can't do is chase a burglar down the street and then shoot him in the back.
First you need a firearm, in your home, loaded or at least with ammunition readily available.
Wasn't there a farmer in Britain who went to prison for shooting a bad guy inside his home? As I recall that didn't go well.