Not to mention the fact that you can't do anything half useful with an NPPL as far as ratings go ..
What you can do :
Section 1 Schedule 8 Part B Section 2 of the ANO
a.k.a. page 240 of this ....
http://www.caa.co.uk/docs/33/CAP393.pdf
And what you can't :
Section 1 Schedule 8 Part A Section 3 of the ANO (p. 235) ...
No night, no IMC etc. ....
As others have said, if your (eventual) end goal is a CPL then a PPL is a worthwhile investment (providing it is done at a good FTO and not rushed).