The example you give is hard to believe (and if it is true please complain, as the crew should be reviewing safety drills at this time). I think it is highly unlikely they were playing games anyway...

The answer to your question "why should I turn of my electronics?", without wishing to sound rude is Because they say so.

It is a safety related instruction not made up by the crew (and in fact the crew hate to have to enforce a regulation that they too know is inconsistent and does not apply on some non British Registered aircraft). They know you are encouraged to use some of these items with US carriers, but like other regulatory bodies (including Japan, China, and some Eurpoean countries), the UK CAA/DOT are still insisting that crew enforce these rules in the name of safety. They are not doing it for fun. They are doing it because it is their job, they are trained to enforce it, and should they be found not to be enforcing it would be liable to action by a CAA/DOT inspector who may be on board checking them out.

By not complying with your crews request YOU are breaking the UK Navigation Orders and are subject to prosecution.

It really annoys me when I see people asked a few times politely by crew to switch items off and then blatantly (and childishly) continue to disobey them. It is their job, and until such time as the rules change you should comply.

Rant over!