ETOPS 90 min certified on A300 in 1976.
ETOPS 120 min certified 1985 on 767-200 and A310
ETOPS 180 min certified 1988 and started in 1989, but initially only on aircraft types with 1 year of trouble free ETOPS 120 min service. Only in 1990 did Boeing obtain permission for the 777 to have it from initial certification.
ETOPS 240 mins from 2007 on the A330
ETOPS 330 mins from 2011 on 777 initially on GE engined models.
The time limits are also based on the single engined cruise speed for each type.
The A319/320/321 only received ETOPS 180 in 2004 and the 737 NG variants in 1999, despite having 120 min ratings since the 1980s.
So when Nimrod MRA4 was selected in 1996 the only twin jet airliner based options from the civil market would have been large wide body airframes, so probably too big for the role.