1. Just to confirm: Esther and Isaac were Jewish (no surprise from the names) and married in 1794:
Year:1794 Supplied Surname:Alexander Surname:Alexander Full First name:Isaac Supplied First Name:Isaac Spouse Surname:Barnard Spouse Full First name:Esther Spouse First Name:Esther Place:Great Synagogue County:London Record source:Boyd's Marriage Index 1538-1840 (As found on FINDMYPAST site).
2. It is possible that either of them might still be alive on the 1841 and 1851 censuses; the latter would help pinpoint where they were born.
There is no obvious couple Isaac and Esther Alexander, but there are several widowed Esther Alexanders and several Isaac Alexanders who rather look as though they might have a second wife.
Do you have the names of their children, as this might help identify them on the censuses? Also Isaac's occupation?