steakandchips, a few errors:
Yes, you can add an SSEA Class Rating to a NPPL/SLMG very easily, but to convert an NPPL to a JAR-FCL PPL is less straightforward as, irrespective of SLMG training time, you must have the requisite time under instruction and under supervision with a JAR-FCL instructor nonetheless.
I have yet to learn whether there will be a direct conversion route from NPPL to EASA PPL; however, there will be a route from NPPL to LAPL and from LAPL to EASA PPL.
However, a JAR-FCL PPL on a TMG, followed by an SEP Class Rating may indeed be less expensive - but the all-in cost needs to be thoroughly researched.
There are no 'cut price' routes to CPL - if there were, then everybody would be using that route. Remember that in aviation, if something seems too good to be true then it probably isn't true!