Interesting post from Airbanda. Only in a classic British state-owned company could so many callsigns be in simultaneous use - in what was (as far as the travelling public knew) a single airline business. Or am I over-simplifying ??
Possibly by the time Speedbird had been universally adopted, British Airways had walked from EGNM and passed their routes to British Midland and Air UK (1980). But they came back- eventually!