IAP: "account not in this store" error with test flight

I've got an existing app that I am trying to add in-app purchase to.


This has worked in testing but I have now submited it to test flight. Testing under my own account, I am getting the message:


"Account not in this store: Your account is not valid for use in the New Zealand store. You must switch to the U.K. store before purchasing"


This seems rather weird. Anyone got any ideas?


I have tried switching to the UK app store and the purchase process seems to go further but I don't really want to purchase the IAP on the wrong store without knowing more.