Surey it is not up to BAA to determine appropriate size and weight of cabin baggage, merely to ensure conformity to set standards. There are precedents here at other airports, for instance at HKG the airport staff are notoriously rude and inflexible when it comes to hand baggage exceeding standard size constraints even for F and J PAX. What happens at HKG is that the check in staff tag the hand luggage as compliant with their own rules (and I have have heard whispers that this triggers a charge to the airline, for each tag). This is not a new thing at Chep Lap Kok either, the same procedure used to apply at Kai Tak in my experience.