(Quite a few 'if's in the following - apologies in advance).
If you have a MFSW and if you don't have Voice Control activated then I believe the default functionality for the microphone button on the steering wheel controls is as a MUTE button. If you don't have MFSW then, if you have a '*' button on the RNS510 (top-right corner on fascia), then (again if you don't have Voice Control activated) this might also work as a mute button (I have read that this might need to be configured).
If you are lucky, you might find that this will mute just the radio without affecting Sat Nav guidance (and without having to twiddle with the volume control).