Correct me if I'm wrong, but I don't think anyone is talking about the UK being unable to access the Galileo signal, at either level of service? I think the issue is sovereign access to the encryption algorithms, the ability to "see inside" the black boxes and to build our own kit rather than buy from the EU. 3rd party access will cost, and obviously it won't be a sovereign capability, so if they decide to cut us off, we're out. But the service will still be available.
Might have the wrong end of the stick on that, but fairly sure I don't!