In regards to the Brexit Deal: If the United Kingdom leaves the EU without a deal you will still, in the majority of cases, be able to use the CE mark to demonstrate compliance with the legal requirements and be able to sell products within the UK market after 12 April 2019. This is intended to be for a time-limited period only. Furthermore, in some cases, as referenced in the document, you will need to apply the new UKCA (UK Conformity Assessment) marking to products being sold in the UK.
To check whether you will need to use the UKCA marking please refer to the guidance on trading goods regulated under the ‘New Approach’ if there’s no Brexit deal.