UK teen who saves his mother from being killed arrested by police

How self defense is treated these days in the UK:

The friend of a teenager arrested on suspicion of murder described him as a 'hero' today for trying to defend his mother from a knife attack.
Schoolboy James Killen, 18, was held by police after next-door neighbour Jonathan London, 46, died of stab wounds.
It is believed Mr London attacked the teenager's mother, former air stewardess Sandra Crawford, at her home in St Alban's, Herts on Friday.
Mr Killen was arrested on suspicion of stabbing to death Mr London but later released on bail.
It is believed the teenager, a student at nearby Sandringham School, attacked Mr London after he discovered him slashing his mother.
Today his close friend Ben Riddell, 18, also from St Albans, said 'kind and gentle' James was desperately trying to defend his petite mother.
Ben, a fellow sixth-form pupil at Sandringham, said James came downstairs and initially thought his mother was being punched repeatedly by Mr London.
He said: 'James is one of the kindest, most gentle people I've ever known. . . .
'Apparently at 8.40am on Friday morning the man just walked in off the street and James came downstairs to his mum's screams and thought the man was punching her.
'He jumped on his back and got the knife and managed to kill him. . . .
Yesterday, an au pair described the bloody aftermath of the fight.
Livia Frakas, a 27-year-old au pair from Slovakia, was taking her employer’s two children to school when she heard a man’s ‘furious yell’ before finding Ms Crawford ‘covered in blood’ outside her £600,000 home in St Albans, Herts.
She said: ‘She was in a nightdress and pyjamas, blood was everywhere, all over her body and her face as well.’
She said a younger man, believed to be James, then emerged from the house looking ‘shocked and desperate’.
She added: ‘He was crying for help. He was in his school uniform. He was on the phone to the ambulance and I heard him give the name of the road.’ . . .