so, if i started NPPL training tommorrow, for example, and got to April next year with, 30 hours, would i not be able to use those hours to complete the LAPL, or would i have to start again?
As far as I am aware, flying training is flying training up to the point of qualifying navex & dependent on how much instrument flying you are taught etc.
Your instructor should be informed as to what licence you intend going for & adjust the sylabus, but there isn't much difference, Group A aircraft that is. The biggest problem is the medical. If you think you need less restrictions, possible IMCR/IR later then it would be PPL. NPPL is restricted to daylight VFR & VMC. UK airspace, though that is changing? etc.
The 40hr? NPPL is unlikely to be 40hrs on the dot. That is only the legal minimum. Though at your age you will/should progress quicker than an old goat like me who took 60hrs.

I suggest you find an instructor who knows what he is talking about!!
Nice day for it today managed to get 1-30min daylight under the wheels.