Flour in UK has to be fortified, it means "by statute any white wheat flour that is milled in the UK has to have calcium carbonate, iron, thiamine/Vitamin B1 and Nicotinic acid added." (Outdated list I think)
Calcium carbonate = chalk.
I feel like when I googled it, all first page was a hit. Though most of them talked about fortification as a whole and not chalk specifically.
That reddit post talks about how that statute (somehow different from law?) is based on outdated beliefs. However, that reddit post itself is slightly outdated, as for example it doesn't include this:
"UK's move to fortify flour with folic acid to help prevent birth defects hailed. The UK government's decision to finally make it mandatory to add folic acid to non-wholemeal wheat flour to help prevent life-threatening spinal conditions in babies is being hailed by all concerned." - 2021
(Though not sure if it doesn't talk about different kinds of flour?)
Ftr, here's the link to the current law of it:
https://www.legislation.gov.uk/uksi/1998/141
(Click "latest available" if it's not on by default)
Essential ingredients still include CaCO3.