Going by memory a caution can only be given if someone at the minimum pressed for charges and wrote a statement. if no one made a statement then it wouldn't matter if the offender even admitted to it in interview it would be NFA'd.
This was reference to a guy trashing a pub and since the landlady only called police but didn't decide to fill a statement out or press for charges even though the offender admitted it no caution could be given.
Domestics are high priority and self defense is an admittance of assault, so now it will be justified in the mags (should have just gone "no comment"). Im pretty sure there has been some tinkering going on with the pol and CPS for them to push for a charge especially if your son is known to the pol.
Let us know how it goes im fairly certain he will win if there isn't even a statement from the victim.