All the answers are correct but it was not so much a case of slow adoption it was a decision by Denis Healey UK labour chancellor of the exchequer in 1975 to adopt the US billion. ie 1,000,000,000 (UK billion became the new trillion ie 1,000,000,000,000). This was not so important when the numbers were unthinkable amounts (except perhaps in science) but when GDP and debt began to get into the realms of this range it became important that the words were universal.