I'm sorry I have no idea what a 'management PG dip in Unitec NZ' is let alone whether it would get you a job. I think if you really try hard at anything useful to you, it can't be a bad thing.
Even with the best qualifications no one can guarantee you will get a job, but it is more likely that you will.
WHV for the UK or PR in NZ?
. I guess you could try either or both, but not at the same time, of course
.. If you are undecided you could spend some time in the UK having a look around if you haven't been there before. It's an opportunity while you are still young. If you find you like it you could consider what you'd like to do next. Alternatively you could try for PR in NZ but you need to meet the strict criteria required by INZ. It's a pity that you can't get a WHV for NZ because you could go there and use the time to apply for jobs as well as travelling around seeing the country. The only other way to find a job would be to go to NZ on a Visit Visa and make enquiries. Some people are lucky enough to get a job offer from outside NZ, but it's not that common as employers usually like to meet you face to face.
I don't think anyone would be willing to decide for you whether you should go to the UK or to NZ. It should be your choice. I would suggest you fully research both countries and the opportunities you might find in them and perhaps the answer will become apparent to you. I hope so.