You’ll find its not BA asking you to switch off your phone from gate to gate, it’s british law which enforces you to do so courtesy of the CAA and therefore we are all expected to follow it every bit as much as we are expected not to rob a bank, drive without a seatbelt or murder someone! Lol!
However BA is working with the CAA to try to encourage a change in the rules to bring it more up to date. However until the date it is permitted it will remain illegal to switch on a phone until otherwise informed so best not to back chat to the crew over the fact!
However there was a fatal accident back in the 90s which resulted in a knee jerk reaction by most European countries to ban the use of mobile phones onboard it’s aircraft which still remains today!