Why customs and when
Customs clearance is required whenever goods cross the EU customs border. After Brexit this applies to every shipment between Poland (EU) and the United Kingdom — both export to the UK and import from the UK to Poland.
United Kingdom / Brexit — ELO, ENS, GMR
For the GB direction you need safety declarations (ENS), ELO readiness and the Goods Movement Reference (GMR) in the GVMS system. As the carrier we prepare the GMR, but we cannot create it without the GB import documents supplied by the sender or client. We make clear who provides what.
Regime 42 (procedure 42)
Regime 42 is an EU import clearance with deferred VAT — goods enter free circulation in one EU country, while VAT is settled in the country of destination. It is used when import is followed by an intra-EU supply (e.g. consolidation from CZ/SK/AT/HU).