If you want to fly with more than one pax, learn to fly at night or on instruments, or fly multi engine you might as well go fo the PPL know. If you just want to fly in decent weather with one other, go for the NPPL.
The NPPL with a Single Engine Piston Land (SEPL) may carry 3 passengers (4 up including the Pilot). The maximum authorised weight of the aeroplane must not exceed 2 metric tonnes. The NPPL can add a night Rating, Aerobatic Rating and also fly 'complex types': i.e. retractable undercarriage, constant speed propeller and also be turbo charged. For most PPLs in the UK the only restriction, that could matter and for most it doesn't, is being restricted to UK airspace although with the exception of the Channel Islands. Very few 4 seat aircraft exceed 2 metric tonnes and only a relatively small percentage of PPLs hold any kind of instrument rating (at least that is maintained current) so cannot fly in IMC either.
For most private pilots in the UK the NPPL is all that they require.