The BA crew contributors told us on a previous thread,they can only ask passengers to remove headphones in an announcement. They would never go up to them and ask. Same with people reading and/or typing on computers.
Personally, I would have no issue with Cabin Crew being given the authority to ensure all pax remove headphones for the safety briefing – supported by the Captain’s announcement.
Again, I am pretty sure the BA members have also confirmed the reason they don’t demand, is the Airline do not want to upset their passengers by making these demands…