As noted by other commenters, some carriers (I think all of the UK carriers) use IP-based geoblocking to ensure that Wi-Fi Calling only works when using an IP address registered in the UK.
For EU carriers roaming used to be a big business (Europe is probably the region where most traveling abroad happens, many small countries close together without travel restrictions). The EU carriers don't seem to be able to let go of crazy roaming charges (5€ per minute and more are not uncommon outside of EU).