Just my humble opinion but I think that an aircraft is 'named' at certification and this will only change when a constructor changes names during the production process. In the case of the BAC 1-11, the company name changed to BAe upon the merger in 1978. BAe still tried to sell newer variants after 1978 and production didn't stop until 1982 but only 8 aircraft were produced and delivered after the merger so they probably didn't bother changing the name. By that time it was well known as a BAC aircraft so from a marketing standpoint you'd want to hang on to that.