Since 31 January 2020, Great Britain is considered a third country for the EU. During the agreed transition period from 1 February  to 31 December 2020, the EU pharmaceutical law will continue to apply. In other words, pharmaceutical companies and medical device manufacturers can resume their activities until the end of the year.
EMA points out that during the transitional period marketing authorisation holders/applicants can still be established in the UK and Qualified Persons for Pharmacovigilance (QPPVs) and pharmacovigilance system master files (PSMFs), as well as quality control testing sites, can still be based in the UK. At the end of the transitional period, all necessary changes to authorised medicinal products must have been made. It must therefore be ensured that they comply with EU law.

The EMA will be publishing updated guidelines on Brexit shortly.

Source

EMA: Press release
MHRA: Transitional period for medical devices