I cannot send an RCS message from my Pixel to my wife's carrier-provisioned RCS app on her Samsung phone.
Google doesn't seem to be interested in a federated and interconnected RCS infrastructure anymore.
In general it's a completely trash fire situation where we're pretty much forced to use proprietary services without APIs to communicate.
I do remember having loads of problems with RCS registration failing on the Samsung side, or just disabling itself.
Edit: Specifically I think it won't register whilst on WiFi and requires the correct mobile APN. I used to switch APNs to get a public IP address/ unfiltered internet
To my understanding, carriers were encouraged to either implement Universal Profile on their own or use a Google-operated "Jibe as a service" infrastructure. The former are now completely isolated from Google's network.
To me it looks like British carriers are simply using Jibe.