Originally Posted by
Ldnn1
Well it could actually be done simply by making a new booking for the whole itinerary and cancelling the existing booking. However that depends on there still being award seat availability for the LHR-BLR sectors. What you don't want to do is cancel that booking beforehand in the hope that your cancelled seats will become available again, as there's no guarantee they will.
This.
It's poor that BA won't link two bookings upon request. (I am loath to say "can't", because clearly it would be possible if BA wanted to). There's not much else to say about that sort of anti-passenger "too hard, meh" attitude.