Good theory...... but not quite right.
Whilst the UK call centre staff are outnumbered by those overseas they are all part of the same "pot".
I can't imagine BT will ever get rid of the overseas call centres altogether as it would be prohibitively expensive. I'd suspect that the wage bill would treble for an all-UK call centre system.
The local BT centre manager tells me that they are currently re-aligning their service to bring more complex "Tier 2" work to the UK with the "Simple" Tier 1 support remaining in India.