Terminal security is generally not the responsibility of the carrier, but rather the owner of the facility. In this case it means BAA. Sorry, but that's how it is right now. And, to make things even more interesting, it's technically not BAA's responsibility either, but rather G4S, who has been contracted to provide it.
Of course, coordination between BAA (the infrastructure operator) and BA (the tenant) would be ideal, but we are not in an ideal world. It's BA's responsibility to notify their customers both land- and air-side that the flights are not leaving and that customers will need to go back to arrivals to receive their luggage back and then go air-side for further information.
Of course, such responsibility will be passed on like a hot potato because "it's not my job", and in the end the pax become annoyed and upset and those who do take the initiative are likely to get a bollocking from not only their colleagues but also their union and their superiors.
S.