Not sure if you were just helpfully including parts of the summary for the benefits of others there? (Full report was actually linked in the road.cc article in a rather unusual turn of events for news outlets)
I couldn't see anywhere in the report a source for any estimate of total distance travelled in order to obtain casualty rates. Couldn't even see an estimate of that distance, they just jump straight from casualties to rates.
Unless they've snuck some out recently I've not seen any from the DfT either. We might get some in the provisional 2021 road safety release which tends to turn up in June (ish) but possibly not until the full release in September.