Although the quote pulled indicates suppliers, it does not say anything about how whistle-blowing is handled when the whistle-blower is an employee.
Yes it cannot be applied to employees as there is no mention in it.
p.2: "The Company has a number of measures in place to identify, assess and monitor its suppliers to ensure compliance with the Modern Slavery Act 2015 and to:
o Identify and assess potential risk in its supply chain
o Mitigate risk of slavery and human trafficking occurring in its supply chain
o Protect whistle blowers"