Just about everyone in the UK defines a billion to be 10^9. I've heard about the 10^12 definition but never encountered anyone who uses it - I think it must have been an older usage that fell out of favour.
In 1974, the UK government abandoned the long scale (million, milliard, billion, billiard) in favor of the short scale (million, billion, trillion, quadrillion) used in the US. The long scale is still used in languages like Dutch, German, and French.