I read somewhere else that the reason was that because the departure retail mall has domestic and international pax mingling together there was a possibility that an incoming pax could enter the country by getting on a domestic flight without clearing immigration. I guess that would mean a devious pax who had earlier arrived on an international flight to connect to another international flight, but had cunningly also booked a domestic flight and printed his/her boarding pass at home might get on that domestic flight and thus join the great mass of IIs.
Manchester and Gatwick take photographs of domestic pax and only allow them to enter domestic gate areas if the photos match.
I guess the fingerprint approach is BA's or BAA's or both to do away with the staff carrying out this function by getting the paying pax to "self serve". I may be doing the bean counters an injustice though, as I suspect the real reason will be to allow flexibility at each gate to handle both domestic or international flights.
I have no problem with this approach if, as with the photos, the record is destroyed after 24 hours. But given the nature of the present security animal, there must be some interest from one agency or another to keep them for a century of two once they get wind of the process.
But it remains a complete mystery me why I get my picture taken at LGW South gates 1-10 as I arrive on a domestic flight, and am only let through to domestic baggae reclaim if the photo matches.... That's blamed on Customs, as I recall.