The GRO index does not show the name of the spouse until 1911, but as each page of the register contains between 2 and 4 marriages (depending on the date) it is possible to narrow down who might have married whom very easily.
If you use www.freebmd.org.uk
to search for civil registrations (which you should do, as the search facility is superior to that of Ancestry who use FreeBMD's transcriptions of the index, so the data is the same) all you need to do is click on the page number and it will take you to a list of those who share the page number. This is the list for the marriage in question:
Marriages Mar 1843
Eyles Robert Southam 16 427
Garritt Mary Ann Southam 16 427
LEA James Southam 16 427
Pratt Sarah Ann Southam 16 427
So not the marriage you are looking for, alas.