Although widely disapproved as nonstandard, and more common in the habitual speech of the less educated, ain't is flourishing in American English. It is used in both speech and writing to catch attention and to gain emphasis.
It is used especially in journalistic prose as part of a consistently informal style.
This informal ain't is commonly distinguished from habitual ain't by its frequent occurrence in fixed constructions and phrases.
In fiction ain't is used for purposes of characterization; in familiar correspondence it tends to be the mark of a warm personal friendship. It has also long been commonly used in popular songs, both for metrical reasons and for the informal tone it conveys.
Our evidence shows British use to be much the same as American.