Policy and legislation are not the same thing. Policy is how cabinet decides to run its ministerial portfolios. If they want to implement a policy that requires large changes, this will often require new legislation, but the two things are quite seperate. The job of the civil service is to implement government policy, which may include the whims of a particular politician. Civil servants are supposed to be independent in the sense that they will faithfully implement the policy of any government, regardless of their own political opinions on said policy. They do not operate independent of government policy. That is how you ensure thousands of unelected government officials uphold the values of a democratic society.