thomasp, you got me there. What I think, but I'm not sure, is that for the NPPL a radio license is optional, but for the PPL it is mandatory. And a radio license is what's required for flying in controlled airspace.
But I'm not from the UK so my knowledge of the precise privileges of the UK NPPL is not a lot. I checked my air law book and it's not sufficiently clear on this issue to be sure. Anybody else knows how this works?
I did find that for the NPPL the min viz is 10 km inside controlled airspace, and 5 km outside controlled airspace. That's more than the VMC minima for a PPL.