What if your BIL just "forgot" to pack it to take it home?
If he's a resident of the UK, that is. If he's a US citizen then he's going to have to pay the tariff.
The penalties are, and should be, set on a curve.
He damaged a plane through negligence, not a deliberate act.
If you throw the book at someone for damaging property without intent, what penalty do you have left to hand out for someone who...