It is a UK CAA regulation for all airlines that operate under their rules, which applies to UK airlines.
This was brought in by the CAA after 9/11, at the same time they brought in the rules about the flight deck door having to be locked during the flight.
The boarding cards on mobile devices have to be created by the Boarding pass app, copies of the printable boarding passes are not allowed to be used.
As crew we have to check the date, flight number and that it has the square barcode as opposed to the long barcode found on a printed boarding pass.