I want to know if this is an American thing or a whether the British feel this way too.
If Reddit is to believed, you should never swap seats on a plane. Not only this, but you should never even ask. It seems like it's a breach of social contract to ask. Not only do the Americans get very upset if they swap, but they often make a stand and then thousands of people support them.
What's the big deal? The other day father asked if he could be with his daughter. One row ahead, identical seats otherwise. I said yes. Some other times I've replied "Can we wait till we're in the air and then swap?", but usually I just swap, a free way to help someone out
(the post was 'Why do people try to swap seats in airplanes?' in NoStupidQuestions)