From Linklaters legal firm website
Jurisdiction over “live” cases during the transition period
The Commission will continue to have jurisdiction over antitrust and merger cases “initiated” before the end of the transition period. In relation to state aid proceedings, the Commission will be competent to initiate (and later determine) new proceedings in respect of aid granted before the end of the transition period, for a period of four years after the end of the transition period.
Binding nature of decisions
Decisions adopted by the Commission or the EU Courts before the end of the transition period, or after the transition period as set out above, and addressed to the UK or to natural or legal persons residing in the UK, shall be binding on and in the UK. The legality of such a decision shall be reviewed exclusively by the Court of Justice of the EU (“CJEU”).
Monitoring and enforcement of commitments and remedies
Unless otherwise agreed between the Commission and the CMA, the Commission will continue to be competent to monitor and enforce commitments given or remedies imposed in, or in relation to, the UK in connection with competition or merger proceedings unless the Commission and the CMA agree otherwise.
QED