I go out of my way not to sit next to work colleagues on a flight! They might want to talk to me and discuss work! I always tell them the flight is full so we can't sit together, how terribly unfortunate.

But if you really want to sit together, I guess I'd call BA Exec Club first to see if they'll link your bookings and allow you to assign your colleague a seat by you. Might as well try that before paying to assign a seat.