Green’s Dictionary of Slang

an excl. used as a response to a statement that the speaker feels to be rude, crude or otherwise unacceptable; sometimes as charming, I’m sure!

1965		J.P. Carstairs Concrete Kimono 232: ‘Tell her to — herself!’ ‘Charming!’ I said.
1967		J. Hibberd White with Wire Wheels (1973) 158: mal: Go flog yourselves. rod: Charming. simon: Lovely language.
1982		J. Sullivan ‘It Never Rains’ Only Fools and Horses [TV script] Oh charming!
1984		P. Barker Blow Your House Down 31: ‘Like if there was a police car there he’d go and park beside it.’ ‘Oh, charming!’.