Should be available at the usual 355 days ahead, like the long haul flights. I have booked overnight long haul flights the second they go on sale, and had to phone up within 24 hours to get a free domestic connection added on. Because of the overnight nature of the long haul the domestic leg is not on sale at T-355, but the day after.
Phone BA once you have the long haul leg booked- and explain that you couldn’t book the LHR-EDI at the time.