Astor: “If you were my husband, sir, I’d give you a dose of poison!”
Churchill: “If I were your husband, I’d willingly take it!”
The joke appears to pre-date Churchill's use of it.
Astor: Winston, you are drunk, and what’s more you are disgustingly drunk.
Churchill: My dear, you are ugly, and what’s more, you are disgustingly ugly. But tomorrow I shall be sober and you will still be disgustingly ugly.