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Like @Icemario said, except a GBP card can't work like that. It's especially hard to change your country of living now, with the addition of using your NNID for eShop. I made a mistake and set it to the UK, so I can either buy a GBP card coupon or use another credit card with GBP as it's currency. In your case, you can ask your parents to use their credit card to add PLN to your eShop account. Trade physical money for digital. Nintendo is a safe company for credit card transactions. In their contracts, if given hard evidence, you can sue Nintendo for theft. It's safe.Check the currency on the eShop, if it's £ then it would definitely work.
Otherwise I don't know for sure but since they're both European it's possible.