As of the withdrawal date, the United Kingdom will no longer be part of the customs and tax (VAT and excise) territory of the Union. Consequently movements of goods between the UK and the EU 27 will constitute third country trade. The principles outlined in this paper aim to ensure an orderly withdrawal of the United Kingdom from the Union in respect of customs related matters. [...] It is for the declarant to demonstrate the status of the goods (Union or non-Union) before the date of withdrawal and the fact that the relevant movement or entry into a customs regime was initiated before the date of withdrawal. The following general rules seek to implement that approach and should apply in accordance with Union Law, as interpreted by the Court of Justice of the European Union on the date of entry into force of the Withdrawal Agreement: ...

No wonder UK gov't hastens to enact the "Repeal" bill (12,000 EU-compliant provisions?) with the Poison Pill (Henry VII).

Retention of existing EU law
2 Saving for EU-derived domestic legislation
3 Incorporation of direct EU legislation
4 Saving for
Signaturerights etc. under section 2(1) of the ECA
5 Exceptions to savings and incorporation
6 Interpretation of retained EU law

EU's workgroup will need all of 2018 and an army to comb out the nits in it.

Diversity is the key to economic and political evolution.