The fact that the goods are being processed abroad appears to prevent the normal temporary exports arrangements from applying.
In Poland, the goods are being imported for “inward processing”. The treatment for Polish VAT/duties will be in accordance with Polish law. I don’t have any knowledge of the Polish arrangements, but here is HMRC’s guidance that would apply were the transaction the other way round (i.e. Polish goods processed in the UK, then returned):
https://www.gov.uk/guidance/apply-to-delay-or-pay-less-duty-on-goods-you-import-to-process-or-repair
You’ll need to establish what the Polish equivalent of this is.
On return to the UK, I believe that you would be liable to import VAT/duty, due to the rules on Returned Goods Relief specifically requiring the goods to be reimported in an unaltered state – see https://www.gov.uk/guidance/pay-less-import-duty-and-vat-when-re-importing-goods-to-the-uk-and-eu
However, the UK import VAT should be recoverable by you as an input to your onward taxable sale of the processed goods .