The more I watch and the more it makes no sense.
The do a late manoeuvre to exit.
The Audi maintains a sensible speed until the exit to allow motorway traffic to keep flowing, once he exits he brakes gently no doubt because the road has a slight curve.
When the Audi brakes you clearly react as you immediately flash him but seemingly make no attempt to slow down or did you flashing him cause him to brake, if so why were you flashing him, was there an altercation earlier on that your not showing us? Had the brakes on your car failed or something or was it more a case your driving like a prat and you thought you'd show him better by not attempting to slow down and instead cut across the front of him, but upon doing so you then backed out of said move as maybe the Audi did not yield or you miss judged and so decided to pull back on but again miss judged and crashed.
You are in control of your vehicle, the highway code stipulates keep a safe distance, you crashed because of your own actions and you seemed to be driving somewhat aggressively, if a car brakes in front of you then your supposed to also brake.
I drive defensive so when cars in front of me brake, even if I have a relatively large distance and don't even need to brake, I still brake and slow down to maintain the gap, its just driving safely.