All policy statements and executive decisions have to be lawful and covered by legislation to avoid 'arbitrary government', a characteristic of dictatorships and authoritarian governments, not liberal democracies.
E.g. during the COVID-19 pandemic, PM Johnson instructed people to stay at home except for a few exceptions, however for these guidelines to have any force, such as fines for those who defy them, parliament had to pass the Coronavirus act 2020.