I was on the LGW/LBA the other day and it was in the 60`s both ways, with only 78 seats id say thats not a bad load!!
Last time i drove LBA/london it took 4.5 hours and that was outside of rush hour and without queues on the M25.
Last time i drove NCL/london it took 7.5 hours.
I would say no comparison, take the plane every time!!!!