Billions and Billions
The following is from the fourteenth edition of The Chicago Manual of Style.

It is important to bear in mind that in British usage a billion is equal to a million million, not a thousand million as in American usage. When writing about British sums for American readers, it may therefore be preferable to use the term thousand million or million million, whichever fits the case, instead of billion, or else to remind the reader, perhaps in a note, of the difference.