Basically, AIUI, the way it works is that you notify Building Control that youre going to be doing notifiable work.
You can either just say "Im starting on dd/mm/yy, and I will of course comply with the Building Regs", and they rock up now and then to check what youre doing/have done.
Or you can say "here is how I plan to comply with the Building Regulations" and they look at your plans and say "Yup - if you do that it will be fine" or "No, that wont be any good", but once you are at the point where the man he say yes, you crack on and they rock up now and then to check what youre doing/have done.
Or you can go the private approved inspector route where they rock up now and then to check what youre doing/have done. And it does look in that scenario as if the inspector has to tell LABC its going to be happening, LABC might want more details than "Wes gonna be doin an extenshun", and your appoved inspector can optionally ask them to assess the plans and issue a "certificate" to say "Yup - if you do that it will be fine".