The answer is posted elsewhere on the BA board, but basically as a GCH you will be able to TCP (link) the bookings by calling BA, and on the phone you will be able to make a seating request that you be assigned seats together. You will not be able to select seats for your colleague online, and if you move your own seat after the assignment is made, you may not end up sitting together.

As I understand it, technically any pax can request a TCP link be added, but only a GCH can request the seating be assigned for linked non-status, non-full fare/F traveling parties.