Hi @gavinwales Are you on the stable version of One UI 5.0 Android 13? If still on the Beta common practice for Google payments,Samsung Pay usually works but aware some had issues. It should sorted out on the official stable release.
On your other point you would only receive a Trade In Discount if purchasing direct from Samsung, do you just mean you purchased the sim from Tesco Mobile?
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 5.0,Android 13 .