I’ve looked at the devices I have. Both a Bose charger for the QC35 headphones and a Phihong power supply have a similar slide in UK plug adapter. The Phihong UK plug is available from:
While the adapter should slide onto the Chime unit I’m not convinced it will lock in place as well as the original part but it may be worth a punt as they are not expensive. The alternative is to use the US adapter with a US to UK power adapter which will be a tad inelegant. I hope this is helpful.