Another UK user here, TA-1024 ( retail version unlocked) Android Version 8.0.0 Security patch level 1st March 2018
it would appear that the updates have dried up.
My TA-1053 in Czech Republic (Vodafone CZ carrier) is also stuck in 8.0 security level 1st March 2018.
My TA-1024 with UK 3 sim updated today from March 8.1. I got the April update, installed that - rebooted and then the May update came available. Installed and it gave the error that it hadn't installed but it had installed correctly. I cleared the framework cache and all is good to go.
IN UK, TA-1024. Have just updated to 8.1 (OK) then did next update and then all I can get is the [No command]. In recovery screen have tried (with SD & SIM cards out) reboot system, wipe cache & data then reboot, but still goes back to No command... Can I get something to load with adb (haven't used before..). Bought direct from Nokia. Help..!
Sounds like you have done as much as you can. Sorry it didn't help. Hope you get a replacement quickly!