A non-existent entity. A term used by Americans to describe the 'Home Counties' English accent.
If one does admit the existence of a British 'accent', then there are literally dozens of variations, according to the variety of forms of enunciation present in the British Isles:
Queen's, I.e. accent-less.
And many others.
And just for the record, those residing in the British Isles generally do not use stereotypical phrases such as 'Jolly Old Chap'. This is the perception of Hollywood, which also has a habit of casting every villain with an English accent.