Or a few miles longer and you are at London Heathrow
I did that route many a time when I worked at CWL and, for a time, resided near STN.
From CWL Airport to the M4/M25 intersection, cruising at 80mph and using the original Severn Xing, was precisely 2 hours so Cardiff Central to Heathrow would be about 2 hours also.
Trouble is that LHR is expensive and a nightmare to navigate, BHX, by comparison, is a pussy cat!