I can't help thinking that BALPA and BA are going to shoot themselves in the foot over this.
For the punter sitting down the back in a 777 all the major carriers are the same - the IFE, food, service, comfort, is all much of a muchness now - If BA go down the route of cancelling flights everytime there is a warning the SLF are going to vote with their feet and change to a US (or other countries) carrier who have Air Marshalls and can ensure a reliable, on-time, service.
BA is on its knees now - the last thing it needs is a reputation for unreliability.