As I recall the expansion of LGW in the 1980s was to meet the forecast growth in demand for operations generally. The planning for a second terminal was included in future plans in the 1970s. Such plans would not have known the predominant airlines so far in the future and could not have anticipated the changes that took place throughout the 1980s. BA were the major player at Gatwick when the North Terminal opened after their take over of BCal. BA did want NT to be a BA sole use terminal but this did not happen.