Why clearance and when
Customs clearance is required when goods cross the EU customs border, on export outside the Union and import from outside it. This covers the United Kingdom after Brexit, as well as Switzerland and Norway, which are outside the EU.
United Kingdom / Brexit, ELO, ENS, GMR
On the GB route you need safety declarations (ENS), ELO readiness and a Goods Movement Reference (GMR) in the GVMS system. As carrier we prepare the GMR, but we cannot create it without the GB import documents supplied by the sender or client. We clearly split who supplies what.
Regime 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 destination country. It applies when import is combined with an intra-EU supply.