There is no official "noble" English accent, like you would have a Northern American English accent or a Liverpool accent. I imagain that it refers to English as spoken by the British nobility - "Upper class" English, so to speak. Maybe someone knows better than me.

During the history of Norman conquest, the upper class which refers to Frenchmen ,speaks the noble English,and the lower class which refers to English peasants, cannot understand the language of upper class.So as to reach the goal of ruling. About their language, for instance, the high class may say "beef" ,however, the lower class may refer the same thing as "cattle".To some extent,through this way they show relationship of master and servant. This just my vulgar opinion, just for your reference.I advice you to check the general survey of the language-changeof UK.