Hey there…some great advice already but here are some other thoughts. The “UK or US” is perhaps far too broad. It may pay to do some significant ‘desk research’ first to identify a small set of potential markets – the US is like 50 countries, focusing on particular states would be more helpful. Targeting the UK from NZ has significant headaches with the timezone being the absolute worst you could hope for.

There are many tools to quickly survey potential customers online – similar to interviews, as already discussed.

Also however you could quickly build ‘painted door’ websites that offer what you are trying to pitch in various guises (but stop short of letting them actual purchase), run short bursts of online/search/social advertising and see how far people go down your ‘sales’ funnel and what interest you can gauge. We used to use this model very often at Humanventures.co to test ideas within a week or so and get very strong signals on viability and interest levels.