If your company is based in the UK and has registered a substance under REACH, the registration will no longer exist after the UK withdraws from the EU. Only an EU/EEA-based company can may hold a registration for a substance under the REACH Regulation.
If you want to continue exporting substances in the EU/EEA after the UK’s withdrawal, you have the option of appointing an only representative to manage your registrations. You also have the option of moving your operations related to the registered substance to a legal entity within the EU.