They have powers under the Online Safety Act to fine Steam £18m or 10% of their global revenue, whichever is higher, if they fail to take down the curator group worldwide. They could also demand the shutdown of UK servers and blocking of Steam in the UK by all ISP's, VPN's and other networks if Steam (rightly) tells them to get lost.

That however would be a massive escalation and would result in both mass protest and worldwide condemnation. Because every PC gamer in the UK will have lost access to all their purchases. I expect Ofcom to not intervene.