I suspect even BA do not know this. Unless every single person who flew with BA was a member of the Exec club they would not have any accurate method of measuring it.
For non EC members, all they have is name and DOB for each traveller. They could try and track this but they couldn't pick up people changing their name (e.g. women getting married or divorced) or differentiate between people with the same name and DOB.
They could track by credit card but that only tells them about the person who paid and anyway people might use multiple cards or change their card completely.
Ultimately that is why companies have loyalty schemes. So they can more accurately track individuals buying habits and try and sell them more of what they like!