As above, they have a power to stop any vehicle any time and any place without a reason.
However, what you will find is it is BEST PRACTICE POLICY to find a reasonable excuse in the first place. But BPP is not of course law, just Constabulary working practices.
Also on a practical note (which is wrong, but sadly a fact of life for our Political Correct society), finding a reason to stop somebody may avoid lengthy police investigations when a coloured chap stresses he was only stopped for being black etc.
Yes the copper may have acted in good faith but it will not stop their Professional Standards Dept to fully investigate which although ultimately will be found innocent, will prevent the copper from moving Constabulary, being promoted, moving job position and in extreme circumstances forcing him to be station bound avoiding public contact.
I am afraid "Life on Mars" policing no longer exists!!!!