Hello,This is very abnormal situation. Parking lock alone, when lever is in P must hold the vehicle on a very high incline. The foot brake must hold the vehicle in place as well even with selector in R. There is a very serious electronic glitch or even mechanical damage in both foot brake and transmission. Since the vehicle is under warranty, please call Service 24H in UK to have a service team check the vehicle at the accident scene.
I drove the car home with nofurther problem and contacted my dealer who has booked it in for an emergency inspection, thankfully the damage was very light.