Short of putting electronic tags on the ankles of BA pilots, it's nigh on impossible for the company to monitor ones travel movements to such a degree. Commuters will still commute and there has been a recommendation in CAP371 long before EASA was created, that suggested that travel time should be limited to 90 mins and this has been causally overlooked by some for a long time.
The only source of information that BA could access with ease, is ones use of their staff travel and especially on BA flights. However if you booked a standby ticket on another airline, would BA have easy access to the listing history without involving the other airline? Those not using staff travel or traveling by road will still have a great degree of carte blanche with how they choose to plan their journey to LHR, with them being totally off the radar, so to speak.
There has been a large degree of trust placed by airlines that their pilots professionalism will extend to how they plan their travel arrangements to work, however trust is really all they can exercise. I can't see this changing anytime soon at BA or any other airline for that matter.