On 1st January 2020 the UK and EU legal systems separated and the UK went in to a transition period and for that year it was business as normal as all EU directives were still valid in the UK i.e. the 2014/34/EU Atex directives was still being implemented up until the end of 2020.
The hazardous area changes are not backdated so all equipment either installed or within the supply chain can still be used.
The manufacturer has an obligation
To draw up a declaration of conformity, or a written attestation of conformity (for components of products), ensure that declaration or attestation accompanies the product; where applicable affix the UKCA marking3 and the specific marking of explosion protection followed by the symbol of the equipment group and category to the product.
Using Atex labelled equipment, by inference temporaily accepts the EU Type certificate but what is not clear is whether the EU DoC, as EU legal document is valid. or must an UK DoC be issued to include relevant to Atex labelling.
Either way in practice it will me little difference to the user as, by either option, they will have fullfilled there duty of care.
Update: the exception to allow CE (Atex) marking in place UKCA has been extended until the end of 2022.
For further information please refer to the relevant government guidelines.