Might have been a trader taking it out for a test-drive. Going by the inscription on the bumper I would say this is a distinct possibility.
Sounds like this is the usual not giving way when someone is trying to make progress, aggressive driving from the car taking the picture, and lack of thought and consideration to fellow motorists.
The statement "Got annoyed when I wouldn't let him" says it all really, in my view if the Pontiac was indicating, which given the glow from the right hand indicator (US Style) I would say he was, seems that the car taking the picture was driving dangerously to block the Pontiac... The real danger here is not the Potiac, but the lack of awareness from the other vehicle.
White left-hand drive Pontiac near J19 clockwise on the M25. Tried to force his way into my lane and push me into other vehicles. Got annoyed when I wouldn't let him and so tried to run me off the road at the junction by forcing his way onto the exit from Lane 2 whilst I was in Lane 1 going on. Failed miserably and ended up slamming on his brakes to cut behind me whilst waving his finger...6 litre engine and the idiot still doesn't know how to control it. Guess he's compensating for an extra-small trouser region...
Pontiac Firebird - cool car!