According to the following extract from the ODO "billion" in BrE used to mean "a million million", but its meaning has changed to the more common AmE usage meaning "a thousand million".
- In British English, a billion used to be equivalent to a million million (i.e. 1,000,000,000,000), while in American English it has always equated to a thousand million (i.e. 1,000,000,000). British English has now adopted the American figure, though, so that a billion equals a thousand million in both varieties of English.
The origin of this usage and its subsequent change, as explained by Etymonline, are from French:
1680s, from French billion (originally byllion in Chuquet's unpublished "Le Triparty en la Science des Nombres," 1484; copied by De la Roche, 1520); see bi- "two" + million. A million million in Britain and Germany (numeration by groups of sixes), which was the original sense; subsequently altered in French to "a thousand million" (numeration by groups of threes) and picked up in that form in U.S., "due in part to French influence after the Revolutionary War" [David E. Smith, "History of Mathematics," 1925].
France then reverted to the original meaning in 1948. British usage is truer to the etymology, but U.S. sense is said to be increasingly common there in technical writing.
As far as I can remember (1980s), a billion, at least in finance, has always meant a thousand million, and I've never come across its older usage.
When did BrE adopted the AmE usage of a billion?
What was "a thousand million" called in BrE when a billion meant "a milliom million"?