I found the christening details on the LDS free site https://www.familysearch.org/
When searching, don't give too many details...less is best to start with. For instance, to find Sarah from the information you gave, I clicked on Search at the top of the page, selected Census and Electoral Rolls, entered Sarah Harvey, birth year 1852 +/- 2 Kent, then click on search. A list with the 1911 census at the top should appear plus Suggested Records listed on the right. Have another go.
You can select Search All Records or narrow the category by clicking on the menu to the left, BMDs, Immigration and Travel etc.