I also don't think there will be a problem with you working in the UK after graduation. You would be subject to various work visa rules but, realistically, they are going to be so short on doctors in a few years time that they will probably be begging you to stay at that point. I certainly have friends with non-EU passports (e.g. India, Singapore) who have progressed into training jobs without difficulty. I believe the selection processes are identical.
I presume you could also return to NZ later on as lots of UK graduates do exactly that. I don't know whether the NZ system penalises people who studied outside the country when it comes to allocating the best training jobs.
The real question is why do you want to come to the UK in the first place? Most doctors are very unhappy here at the moment and many are desperate to get out and move to Australia or NZ... A lot of things can change in 6+ years but a NZ passport and a UK medical degree ought to give you plenty of options in terms of places to work.