He’s got a point, though. Right now in the UK, if you try to help someone being assailed, you are likely to wind up in prison with a LONGER sentence than the assailants.
Nice, huh?
From my own perspective and of course I cannot say this is what Boris was saying or thinking.
If I saw an attack take place I would try to get assistance to the victim in the way of the emergency services but it can sometimes be fool hardy to give physical assistance you are no good to help someone if you either badly injured or dead.
Though what I would add in most cases of crime of the nature that Boris is talking about why were you there in the first place.
Often it takes places in areas and places where I and my friends and family would not be in the first place. Most people in my experience of crime apart from when you are attacked in your own home are either with people or in areas that they should not have been and would have been advisable to avoid. Maybe this is partly what Boris was thinking about in his advice to his children.