It might be down to the discretion of the station manager. Had a friend heading to a stag do in BCN a few months ago. Needless to say, major traffic problems and the 40 minute journey turned into 2 hours and he missed the flight. He rung BA on the road and they gave him the 'tough luck' speech and that he'd have to fork out 500 quid for a COW!
At the airport, he asked and they said no problem and put him on the next flight.
BTW, what the heck was wrong with the M23 this morning. I was at the M25/M23 junction at 8:20 and it took a further half hour to get to LGW. Was there an accident or something??/