Hi Evianyu,
A seller in Poland needs to speak to their VAT department for advice about VAT exemptions, but importer in the UK will have to pay the customs charges as per the trade tariff.
As the trader is no longer VAT registered in the UK, they need to ensure they have new EORI number.
EORI number issued with VAT number should not be valid or used.
Trader can contact EORI team using below link.
Get an EORI number
Thank you.