This rule is known as Right-Dislocation; it moves the subject to the end of the sentence, leaving behind a coreferential pronoun. There is a corresponding Left-Dislocation, as well, that inserts a pronoun as the subject. Both constructions are accompanied by specific intonation contours, marked with commas; more on these syntactic rules, and similar rules of English, on page 4 here.
- It looks easy, that shot. ~ That shot looks easy. ~ That shot, it looks easy.
The beginning and the end of a sentence are the two most prominent positions available; thus there are dozens of English syntactic rules that have the effect of moving some important word
or construction to either the beginning or the end of a sentence, under specific circumstances.
Dislocated constructions, like most constructions that indicate speaker attitude by intonation, are features of speech, rather than writing.
Oh, and dislocation is not a feature of British English. It's all over the US, for instance.