*General practice
There are some parts of the job that can be/are done by an NP. But I think, as has already been said, there is smaller scope for it due to the way in which general practitioners work in the UK. Here, GPs would be expected to instigate (hospital level) investigations and manage up to the first, second, even third+ line treatments before referring to a specialist. This combined with the fact that most GPs have a special interest which allow them to perform near specialist standard (if not at that standard).
NPs could be used to triage a dynamic appointment list, allocating appropriate appointments where indicated while dealing with the time wasters.
But no, an NP could never replace a GP as they are the gatekeepers into very expensive and high demand secondary care. We need highly trained/experienced doctors on the front line to manage problems in the community as opposed to overwhelming the secondary care system with endless referrals.
I suspect NPs would be a false economy.