What does that mean for non-EU/EEA company who holds an EU REACH registration by appointing OR (not GB-based)?

In case that this non-UK company has or would have a UK client (UK importer) who neither holds an EU REACH registration nor intends to submit HSE a DUIN by 27 October 2021, non-UK company will require two ORs (from 1 January 2021, double obligations): one in the EU/EEA according to EU REACH, one in the GB according to UK REACH.

Under UK REACH, deadline for the full submission of data to underpin registration dossiers will be staggered over a period of 6 years. These deadlines start from 28 October 2021, the end of the DUIN submission period. To benefit the registration transitional provisions, it is strongly recommended that non-UK company should appoint a GB-based OR as soon as possible to submit DUIN to HSE on schedule (by 27 October 2021), on behalf of his GB-based importer.

A DUIN is not a pre-registration. The concept of pre-registration under EU REACH does not exist under UK REACH. After having notified under Article 127E of UK REACH, the DUIN submitter is not obliged to register unless he wishes to continue importing the notified substance after 300 days plus either 2, 4 or 6 years of the end of the transition period.