The syllabus for the NPPL and the JAR PPL are for all intents and purposes identical. There are two points in your training that will decided which of the two paths you are going to take.
1) First solo. If your first solo is on a HGV medical (i.e. NPPL medical) then you are then limited to completing the training as an NPPL. However, if you hold a class two medical then the point at which your decision is made is....
2) First solo Nav. There is a navigation skills test that you need to pass before you can go on solo nav's as a NPPL. If you sit this test before you first solo nav, then you can only complete your training under the NPPL system.
If you have held a class 2 medical since you first went solo, and you haven't sat a Nav skills test, then as far as I can see you can complete your training under the JAR PPL system, with all of your training counting towards the 45 hours required.
If not, then you will need to complete the NPPL and then convert. The details of which my esteemed colleauge BEagle has generously provided.
NPPL + JAR + CAA + PFA = Minefield.