I'll stand (sit) corrected here, but from what I know the main differences are;
1. UK FIR only - no foreign travel
2. No other ratings until JAA PPL. ie You'll be a Day VFR pilot. No IR / Night etc.
For those wishing to fly purely for the fun of it, I reckon the NPPL is the quickest way into flying, and with only 32 (I think, not 35) hours reqd it is much cheaper than a UK based JAA PPL.