If you represent a non-EU company and you are based in the UK, you will no longer be able to act as an only representative after the withdrawal, unless you relocate to an EU-27 or EEA country. Otherwise, to remain legally on the EU market, the non-EU manufacturer that appointed you will need to find a new only representative, based in one of the EU-27 or EEA countries. The registrations will have to be transferred to that new only representative.