By area, New Zealand is slightly larger than the UK (268,000 km^2 vs 243,000 km^2); the two countries also have similar climate.
However, the UK has 65 million people, while NZ only has 4.5 million.
Is it at all possible that NZ can have more people than the UK in the present day, with a PoD no earlier than 1600?
Bonus points if this can happen without a nuclear holocaust.