Oh stop being so melodramatic Juud
In the UK cabin crew can't go round arresting passengers or restraining them at will! You have to get the Captain's permission to do any of those things and even to give them a 'yellow card' warning. And I'm afraid I haven't any intention of giving such permission for a pax who simply won't follow an instruction to stop watching their PDA!
The unbelievably tiny risk associated with such actions doesn't warrant escalating the issue to such a level.