Now on the 3rd Beta Android 11 release for the U.K.
To be eligible for Beta firmware you must fulfil the following criteria...
1. Be in a participating country (U.K. is one)
2. Have a an S20 device (not S20 FE)
3. Be on a non carrier branded firmware (In the U.K. this is CSC code BTU)
4. Have signed up for Beta firmware in the Samsung Members app (Sign-up will only be shown if you meet the above criteria)
To check if you are running non carrier branded, stock, Samsung firmware with CSC code BTU, go to...
Settings > About phone > Software information > Service provider software version... check your CSC code as per screenshot...
AFAIK, there is no way of changing the CSC, (“Consumer Software Customization” or “Country Specific Code“), unless you root your phone. ***
Competent and experienced users could download the Beta firmware and flash it to their device at their own risk and with the possibility of voiding their warranty. See... https://forum.xda-developers.com/galaxy-s20/how-to/oneui-3-0-beta-s20-ultra-beta-thread-t4176833
*** I have been told that SamKey can change your CSC code without rooting. I have not tried this myself. See... https://www.samkey.org/