The difference between nurse practitioners and doctors is something that has been much discussed already on TSR.It will vary hugely between different contexts and different levels of doctors.The role of nurse practitioner is not something that is directly recognised by the NMC, per se. The only thing that officially appears on the register is that the nurse has achieved their supplementary prescribing qualification - something which is generally included in a nurse practitioner course.
Obviously as they are on the register, a nurse practitioner must work within the NMC Code and Standards for Competence. I believe the rest of the monitoring is left to each nurse practitioner's employer. Given the varying contexts nurse practitioners can work, this is potentially the best way of doing it, as long as they are monitored closely.