I'm very happy to be corrected, but when I looked into upgrading an NPPL to a PPL, you could only do so if you had a JAR rated instructor for your NPPL. When I asked around, the NPPL instructors round here who offered cheaper training (e.g. on C42s) weren't JAR certified so you would have to do more than the extra 15 hours in order to upgrade.
Something to be cautious of and bear in mind.