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What sort of works would be required to give BA what they want and cost wise would it be big.
The terminal was devised in era when people actually put their luggage in the hold where its meant to be !
So no stupid mile deep security areas were ever envisaged !
The design was such as to reduce kerb/aircraft paths to a minimum, with customs/immigration on separate floor for ease of flow.
The idea of common airside for domestic/international was largely pie in sky (it exists today for commercial/shopping however does result in the so called need for uptime further security checks downline)
This terminal as is needs an extensive rethink to even come close to current security demands and it ain't simply in the floor space.
Adding further domestic channels and requisite sterile paths makes it even more difficult to meet Home Office/DFT standards.
Pretty much the same can be said in reverse for T3 a primarily domestic terminal forced into multi use for commercial reasons.
If people, airports and indeed so called security experts really was to tackle the queues and especially after the Brussels incident recently we need to force passengers to once again have their luggage (ALL apart for a hand bag/Laptop)
checked and screened and placed in the hold period.
This would return some sense of normality to the departing experience .