Earlier quoted context omitted.
RCS is a federated protocol. Any app could use it if the network providers supplied a service for it. Google is providing a Google server fallback for if they don't. There may not be a UK RCS market, but given it was just turned on a couple months ago and for a few countries, perhaps it's too soon to call it decided?
My point was specifically in response to Google's claim that RCS is a "gold" market for RCS where they have already completed the rollout. They haven't. They have the support of one carrier with roughly 20% market share, and about 3 - 5% of Android devices. There isn't really any reason to surmise that is going to improve, and Google's boasting here is very silly. RCS is a dead end product in every non-US territory,…
According to Google (in the articles I read and shared here), whether the network provider supports it is irrelevant because they are supplying their own server in those cases, so it should just work. If you're in that region I'm interested to hear what you experience is if you have access to an Android phone and test.