Can't you book the LHR NAS and then call after the weekend and ask the Res Agt to 'extend' the booking to GCM. I worked in res (not BA) and if this situ occurred (which it did quite often), we simply booked it as 2 sectors and asked route control to 'combine' the sectors into one. They only refused if it resulted in a loss of revenue which in your case, I don't see that it would. Worth a try.