I have a Pixel 9 Pro and T-mobile service in the US. I wanted to try the IMS technique. However, apparently T-mobile has roaming agreements with every UK carrier as i could not get a carrier to fail registration. I had bought a Vodafone UK eSIM for data as it was only 27 GB for 10 UKP and unlimited UK calls.

As the "pick a carrier that won't register" trick wouldn't work, I went into my profile on the T-mobile website and turned off all international roaming. Boom, the phone now says T-mobile Backup Calling and incoming and outgoing calls are working fine.

Seems to me if you have a carrier that has roaming agreements with all UK carriers but no ability to turn off roaming you are not going to be able to use the IMS technique in the UK.

(Profile:International:Block International Roaming)