Thread is not going to be too helpful to others if you can't even give an indication of what the goods are. The fact that they may be zero rated according to the com code still means there is a duty implication, albeit zero, but docs still have to be raised upon import in the customs declaration. The export declaration should indicate this and indicate if Greek EPA was included. Perhaps your shipper will cover both declarations along with RoO details.
Brexit has nothing to do with imports from outside the EU 27. The extra effort into collecting VAT and abolition of Low Value Parcels is an HMRC effort as they need the dosh.....
Indeed, pre Jan 1st it was in the CU, now GB are a 3rd country (and customs has nothing to do with the TCA deal). If VAT/GST was paid at source outside GB nobody in HMRC/BF is going to care two hoots and UK VAT will be applied as per normal. Greek EPA is 23% odd and, if it was charged, can be reclaimed once shipping and import declarations are made and goods are with the purchaser but as the OP is not VAT registered it'll have to be on "Tourist terms" rather than any Incoterms/8th Directive and there is no compulsion upon the vendor state to receive or issue documentation in English.
I imagine all with go smoothly although there have been instances of the GB authorities not quite being up to task on many import issues - "how much can we levy on these goods" type of attitude.