Problem with leaving the funds in a UK pension fund when you have the option to get them out is that you are bound by the rules of personal pensions - basically you can have a third as tax free cash and the rest has to go to buy an annuity.
Annuity levels have been dropping like a stone over a number of years due to the combined effects of gilt rates going down (which means that the two thirds which is invested produces less) and life expectancy going up (which means that the company providing the annuity expects to have to pay you for longer and as a result gives you substantially less each month).
It is possible that people in their sixties will have had their potential pension REDUCED for each additional year they worked and contributed as opposed to retiring at 60, notwithstanding the fact that they continued to contribute to their fund. The additional amount they were putting in wasn't enough to make up for the reduction in what they were getting back in pension payments as a result of the above factors.
The tight conditions surrounding how you cash in your pensions in the UK is a quid pro quo for the (not inconsiderable) sweetners the government gives you to put money in in the first place - to put in 10,000 pounds only costs you 6,000 if you're a 40% tax payer.
Taking advantage of the option to transfer your pension on migration allows you to benefit from the tax credit whilst freeing yourself from the worst effects of the pension regulations - in NZ you might have to lock in your fund until you're 60 but after that it's yours to do with as you wish. If you prefer to fund a couple of rental properties rather than buying an annuity then you're free to do so. And if you die a year after drawing down your pension you don't lose a hugh percentage of your fund to the insurance company.
You also benefit from differences in taxation by transferring - in the UK your contributions and fund growth are tax free and you then pay tax when you draw down the pension - the annuity is treated as taxable income. In NZ you pay contributions out of taxed income, and pay tax on the growth but are given the funds totally tax free at drawdown. When doing a transfer you can benefit from many years of tax free investment in the UK by transferring it into a NZ without paying any tax. You're then taxed on future growth, but at drawdown the whole of the pot is available tax free - quite a benefit IMO.