there's no law, but publishers tend to choose a house style for the sake of consistency. The Guardian's is the same as the Telegraph's:
"Spell out from one to nine; numerals from 10 to 999,999; thereafter use m or bn for sums of money, quantities or inanimate objects in copy, eg £10m, 5bn tonnes of coal, 30m doses of vaccine; but million or billion for people or animals, eg one million people, 25 million rabbits, the world population is seven billion, etc; spell trillion in full at first mention, then tn; in headlines use m, bn or tn "
The Nox Publishing House works to different rules, especially if it demands that you spell out forty-two thousand, six hundred and seventy-five and a half.