The UK used to require bread to be 400g or multiples (so a regular sized loaf of bread is 800g) but it eventually got rid of that rule. Obviously years ago (this rule was hundreds of years old) it will have been some amount in an obsolete unit rather than grams, but once metrication happened the bakers picked 400g as the replacement size. I'm told it got rid of this law a few years back, but certainly last time I bought a loaf of bread it was still 800 grams.