Which Nintendo eShop card works with your account, and why a European card may not work in the UK.
Nintendo eShop cards are tied to the country of your Nintendo Account. The console itself is not region-locked for most games, but the eShop and its funds are.
Sign in to your Nintendo Account on the web and open the user information page. The country of residence is listed there.
Changing the country of a Nintendo Account can affect your balance. Spend it before you change.