Just remember -
PPL - Allows you the privileges therein
CPL - Required for ANY flying that will result in you being financially rewarded for it. Technically, if you take three friends flying and through all their inputs you will end up, say, 50 pence better off, you need a CPL. Extreme example, but just so you understand.
Instructor's Rating - This makes it legal for you to safely and accurately teach the PPL syllabus to others. You don't need a CPL to get this rating, but you just wouldn't be able to earn money unless you had the CPL.
So what I described aren't necessarily stages. They are all separate, but added together allow you to earn money as a flying instructor.
Any other info, do ask
