There are lots of phones listed on Amazon all with varying prices, a lot of the cheaper ones are imported and more often then not say this is the description, a lot of the cheaper s21 Fe 5g versions I was looking at stated German/Greece etc.
Due diligence is needed when buying phones especially from a global marketplace, I specifically searched
S21 FE 5g UK version
And made sure it stated UK version in the title of the advert, I also made sure I got one that was supplied and shipped by Amazon as returning it in the event something's wrong is pretty straight forward.
It also doesn't hurt to ask Samsung themselves, I used the ask a question within this app and spoke to customer services on the phone who were more than happy to check and clarify the phone was going to work fine in the UK. (Once I received the handset for piece of mind regarding firmware/security updates)
If you didn't buy it direct from Amazon then ask the seller for a return, if that fails have Amazon call you and request a A to Z guarantee dispute/return.