In representations or imitation of upper or upper-middle-class speech (originally that of ‘Sloane Rangers’): yes. Also as noun: an utterance of this.
Of, relating to, or designating a person who is conspicuously or unselfconsciously upper or upper-middle-class in speech; ‘posh’, ‘Sloanie’.
1980s; earliest use found in The Times. From OK + yah.