Since this is Britain, are the passengers going to be prosecuted for restraining him?
The airline is UK, the source airport (Cardiff) is in Britain, but the destination is in the Canary Islands, which are Spanish. The police who took the "disturber" into custody would be Spanish police.
But the passengers should be OK, even with the Brits, as long as they did not use any "offensive weapons", which can include a steak knife. Maybe even a plastic fork. I'm sure beating him with a seat belt extender would be considered use of an offensive weapon.