Been on the U.K. Beta test firmwares for over a month!
You are too late I am afraid.
Samsung have now closed applications for their Beta programme.
You probably were not eligible unless your device is a non carrier branded one on CSC code BTU. In which case you would have received an automatic invite in the Samsung Members app at the beginning of October.
This is the S21 section of the forums and Beta sign up has closed after a month.
See the post before yours.
The Beta programme is only available to stock, non-carrier branded phones on CSC code, BTU.
Is your device on CSC code BTU?