Annual Slavery and Human Trafficking Statements must detail each company's efforts to eradicate slavery from its supply chain as well as its own business.
A company could simply say that it has done nothing about slavery in its supply chain, but that's likely to lead to negative publicity. So most large companies can be expected to take some steps to engage their suppliers.
The Government expects pressure from the media, NGOs, employees, customers, shareholders, and indeed competitors to drive companies to demonstrate progress year on year in a race to the top (and drag their suppliers - the smaller companies - up there with them).