By the end of the transition period on 31 December 2020, 80% of the REACH registrants in the UK completed the transfer of their registrations to the EU. The 20% remaining will be revoked.
For transfers which have yet to be completed, EU successors are requested to accept them as soon as possible. If not finalized by 31 March 2021, the incomplete transfers will be cancelled and the registration revoked, meaning that the registrants will no longer be able to legally place substances on the EU market.
Check out the list of substances registered by UK companies here: https://echa.europa.eu/documents/10162/13552/uk_only_reg_en.xlsx/8065c461-f576-be5c-d450-eb480737de7d