Who has the right to double park.?

So you are driving through a city and a vehicle has double parked blocking the flow of traffic.
now sometimes its a fire engine,or an ambulance,or a postal van,or a delivery van,or a taxi,or a bus,or a guy with a disabled passenger on board,or a car with doctor sticker,or
THE WHITE VAN MAN,or it could the queen,or it could be an army vehicle,or a member of parliament,then you have a guy dropping an old person off,or sometimes its a window cleaner with a high ladder on the back of a lorry.but they all double park as if they have right to do so.
but the question is  WHO HAS THE RIGHT TO DOUBLE PARK?.I find it difficult to understand how a THE WHITE VAN MAN is covered to double park.