What are these terms "provisional headline inflation" and "imputed index" used to predict in terms of inflation?
Headline inflation is the raw inflation figure reported through the Consumer Price Index (CPI)
Raw means it is not adjusted in any way (like core inflation that excludes energy and food prices). Provisional headline inflation just means the numbers are not final and subject to change.
If some prices are missing statisticians will impute them (figure out what they likely should have been) using statistics, like in this report.