For products already covered by Energy Labelling, it is not allowed to 'provide or display other labels, marks, symbols or inscriptions' which do not comply with the applicable energy labelling requirements, if doing so would be likely to mislead or confuse customers [...];
Energy related products bearing both the UK energy label and the EU energy label meaning having two visible labels side by side can mislead customers and are thus not compliant with the Energy Labelling Framework Regulation as this practice violates the anti-mimicking provisions under that framework.
For products not covered by Energy Labelling, it is not allowed to supply or display labels which mimic the labels provided for under Energy Labelling regulations;
Energy labels fulfilling the requirements from a third country would be in breach these of obligations.
Disclaimer: Please note that the European Commission cannot provide a legally binding interpretation of the EU legislation, as this is the sole competence of the European Court of Justice. Any remarks from the European Commission services are without prejudice to the position the Commission might take should related cases arise in a procedure before the Court of Justice.