If I understand the press article correctly, the NZ government isn't preventing Les Munro selling the medals, but it may refuse them an export licence. That seems to me indicative of an understandable desire to keep the medals in the country - would we want to see important medals awarded to Britons being exported to another country?
I don't know how the NZ licensing process works, but I suspect that it differentiates between Les bringing them to the UK temporarily for a function and a purchaser exporting them permanently.