@Solarfire Would hold off for now, hopefully the S21 Ultra will be available at a later date, you could have a look at the S22 line up. There is one on Amazon but only the 128GB storage version seems to be a new Device. It does say Uk version but cannot always be sure. https://www.amazon.co.uk/Samsung-Galaxy-Smartphone-Android-Phantom/dp/B08QNDWYMP?th=1
I do not work for Samsung or make Samsung Products but provide independent advice and valuable contributions.
My Device- S21 Ultra 5G ( SM-998B/DS ) CSC= BTU , 12GB/256GB
One Ui 4.0,Android 12 Official Version.
"It does say Uk version but cannot always be sure."
As JAMES4578 states, it could mean that is a U.K. version but carrier branded for EE/Vodafone/O2/Three. This is different to being carrier locked which is now illegal in the U.K.