@Peterdm: Updates are rolled out in batches based on factors such as location, purchase date and network provider, and will reach some customers sooner than others. All I can suggest is enabling auto updates via Settings > Software Update > Auto Download Over Wi-Fi, to allow you to grab any updates as and when they becomes available to you. Please can you also head to Settings > Biometrics and Security > Other Security Settings > Security Policy Updates, and make sure that this is toggled on.
AUF6 firmware for the carrier branded S21 devices on the the THREE U.K. network has finally just been released for CSC code H3G.
It is a FOTA, (Firmware Over The Air), update and you can check... Settings > Software update... to see if it is available for you. Alternatively, connect your device to Samsung's PC suite, Smart Switch.
(Stand by for 100's of over eager posts about this! 😉 )