You know they are structually joined and before the airside move when land move you could walk between the two (and get nice hotdog on the ground floor of T1a). They built a wall and it really is little more than a partition, indeed there are fire doors still there through it nicely concealed with some cladding as well.
Seriously it would cost no more than a few hundred thousand tops to remove , remodel and restore; wouldn't even need much steelwork..
There a surprising number of the original T1 structures simply hidden behind facades !