And is it totally reliable?
The R rate published by SAGE is based on historic data (hospitalisations, ICU admissions and reported cases) so while technically it may be reliable, it is out of date and applies to infections from the prior week.
I am hoping SAGE and the government get a preview of ONS survey results before they get published on Fridays. ONS combined with Zoe data are probably two best real-time indicators of what is happening with the spread.
There is also REACT (the Imperial study) but it was wrong twice over past 6 months. Once it predicted flattening of cases in September (which didn't happen - infections kept rising) and the second time it significantly overestimated the daily transmission rate prior to lockdown 2.0. I guess the Imperial's sample is less representative than ONS's.
ONS and - to a lesser extend - Zoe have not had such obvious glitches and appear to provide "directionally correct" view of the COVID spread.