Strafer,
You say these things because you are nervous and have never witnessed any of this before. That is understandable. That is not how it would play out. From what I know, everyone would have to obey their orders, and nobody could "sneak up" and bang him/her on the head. Not going to happen. You are making things up in your head because you do not like the idea. Do you think there aren't any current air marshals on US airline's transatlantic flights? No? Have any of them ditched in the Atlantic? If the UK airlines want to fly to the US, then they will comply. Good day!