We can confirm that your understanding is correct in terms of the record keeping requirements and the requirement for making a full export declaration by company A. A customs declaration will be required for the goods that are being imported to the UK and an export declaration will be required for the goods that are leaving the UK. If the value of commercial goods exceeds £1500 (at import or export) in GB, a full declaration will be required. You can find detailed criteria when a full import or export declaration will be required when moving commercial goods as accompanied goods here:
Bringing commercial goods into Great Britain in your baggage
Taking commercial goods out of Great Britain in your baggage
The UK is no longer part of the EU and businesses are required to meet the relevant customs requirements for record keeping, declaring the goods and paying the duties where applicable when moving their goods between the UK and the EU.
If the whole consignment of goods is eventually taken out of the UK, a duty repayment claim may be considered by the duty repayment centre under rejected imports provisions provided the relevant evidence is provided. Duty repayment claims are considered on case by case basis and they are processed and authorised by the duty repayment team who can be reached (externally) at: firstname.lastname@example.org
or 03003227064 (option 2). You can find the process of submitting a duty repayment claim for full import declarations here:
How to apply for a repayment of import duty and VAT if you've overpaid (C285)
When goods move between the UK and the EU they can benefit from duty relief at import under the EU-UK TCA if the goods meet the relevant rules of preferential origin.
Rules of origin for goods moving between the UK and EU
Company A may want to consider claiming preference (customs duty relief) on goods when they are imported to the UK if goods are of EU preferential origin and they meet the relevant origin requirements. This will not remove the need for record keeping and making customs declarations.
Company A may want to consider storing their goods in the UK on long term basis in a customs warehouse which suspends customs duty and import VAT while goods remain in a customs warehousing (storage) procedure. If goods are sold, they can be removed from the customs warehouse and declared to free circulation (through another declaration) paying all the duties if applicable. Preference under EU-UK TCA can still be claimed when goods are released to free circulation from a customs warehouse. Goods that are not sold can remain in the customs warehouse and no duties will be payable if they are re-exported from the UK. Using a customs warehouse will not reduce the requirement for record keeping and customs declarations. A full import declaration is required when declaring goods to the customs warehousing procedure. You can find detailed guidance here:
Pay less or no duty on goods you store, repair, process or temporarily use