You need to look at where the advantages of the NPPL lay. In the main it has allowed those who can't obtain a Class 2 medical to continue flying, in some cases it has reduced the cost of medicals for older pilots. It has also allowed easier conversions from gliders microlights etc. The original concept of the NPPL was to produce a cheaper PPL, achievable in less hours; but this has not proved to be a realistic concept for the majority of ab-initio students, who lets face it are relatively small in numbers. From an examiners perspective, the test standard is the same and achieving it will cost the same whichever route you take. Subsequent conversion will inevitably lead to increased cost and negate any small saving you might have made.
Realistically, if you are training on SSEA, you can leave the decision right up to the point of the qualifying cross country, for the NPPL you will have to take the NST first, for the JAA PPL you will not. Provided you have trained with a JAA qualified FI, conversion is relatively simple and amounts to little more than retaking the Skill Test; if you train with a non JAA FI (SLMG Microlights etc) conversion will cost much more as you have to fulfil the instructional time with a JAA FI.
There will also be a conversion to the EASA LAPL at some stage and will probably be essential.