Until the 1970s, common usage in the UK was that a billion was 1,000,000,000,000 - that is, a million million.
To avoid confusion, primarily regarding finance, the UK official declared that a billion would be 1,000,000,000 - a thousand million - just like the USA.
What is a billion?
What constitutes a
billion is a source of occasional confusion. In official UK statistics the term is now used to denote 1 thousand million – 1,000,000,000. Historically, however, in the UK the term billion meant 1 million million – 1,000,000,000,000 – but in the United States the term was used to refer to 1 thousand million. The US value had, however, become increasingly used in Britain and the Prime Minister, Harold Wilson confirmed in a written reply in 1974 that the meaning of “billion” would be thousand-million, in conformity with international usage.
The Oxford English Dictionary explains why UK and US usage differed.
The text of the 1974 Harold Wilson PQ:
This definition of a billion is now known as the short scale – where each new term for a number above a million is one thousand times greater than the previous one. The historical definition of a billion is now known as the long scale – where each new term for a number above a million is one million time greater than the previous one.
https://commonslibrary.parliament.uk/research-briefings/sn04440/