Hello,
I think the Bixby Text Call feature on One UI 5.1 has been restricted to only US and Korean models, so this is why an error shows up.
Will you please fill an error reporting in the Phone application?
Phone application > 3 dots > Settings > Contact us > Error Report. What matters the most is this process being performed within 3 minutes from the error appearing. Also attaching this screenshot will be helpful for sure.
In the meantime Bixby Voice now supports On Device mode also for UK English: give it a try! 
See you soon!
