I had a car door open up on me when I was cycling past at 25mph, put me on to the other side of the road and I had to go to hospital. There where no witnesses but my claim went through and I got about £3k for it.
This was for cyclists but the main part is relevant for anyone, it's basically illegal to open a car door that endangers anyone else.
the-laws-of-car-doors
In accidents involving bikes and motorcyclists.. most often this will go in favour of them as they are perceived as being more vulnerable road users.
So a car door opening into a car will not be dealt with the same as a car door opening into the path of a cyclist or motorbike.. even though the phsyical act is the same unfortunately