The big difference is the exams.
From memory, the NZ CPL involves something like seven exams (don't quote me on that). These are self-study, reasonably but not desperately demanding, and can be taken at a local examination centre.
In the UK, you are looking at nine exams for a CPL, fourteen if you want to go for an IR/frozen ATPL. This includes a period of mandatory ground school and the exams can only be taken at Gatwick (I believe). Intensive self-study will take a couple of years; a full time residential course will take between six and nine months.
So, apart from the 25 hour flying course and the cost of the CPL test, there is a lot of extra work involved in the conversion. I can't comment on the IR.
Put simply: if you want to fly commercially in the UK, do your CPL in the UK. If you want to fly in NZ/Aus, do the CPL down there. Forget about cost, think about time.