With an extra constellation comes a more reliable and robust solution, with more pseudoranges being added to the equation. In fact, where a GPS-only receiver should see >5 satellites, 100% of the time, a GG receiver will see >10 satellites, 89% of the time.
As GLONASS is inclined at 64.8° to the equator, (as opposed to GPS at 55°) your DOP values will be greatly reduced for higher latitudes i.e. the UK (50° - 61°North), particularly Scotland (GPS only DOP=1.85 Vs. GG DOP=1.35). Further to this, your PDOP will improve by ~25% in the UK.
Leica have published a white paper noting that “SmartNet sees significant benefits above 50°N when incorporating GLONASS”.
In terms of orbital characteristics, GLONASS has a different repeat cycle, which importantly changes its sensitivity to multipath (whereas GPS-only multipath repeats -4mins / day). “The systematic translation of GPS satellite-related errors (as a result of having the same ground-track) doesn’t happen for GLONASS.” With a repeat cycle of 8 sidereal days, it is unlikely your Rover will observe the same GLONASS satellite during a survey, ensuring any satellite specific errors are not re-observed.