For a full description and clarification on this topic, please use the link below:
https://www.gov.uk/guidance/placing-manufactured-goods-on-the-eu-market-from-1-january-2021
If you’ve already placed your goods on the market, in an EU country, or in the UK before 1st January 2021, you do not need to do anything if you use an EU Notified Body.
For products already placed on the market, CE marking will continue to be recognised in the UK market until the transition to UKCA Marking is mandatory. For new products covered by a UK Designated Standard, new marking will be required with immediate effect.