I meant to say that I've looked for Ruth b 1821 in England, with no success.

I put just "Ruth" / no surname, born 1821 plus -or-minus 5 years, Isle of Man, into the 1851 and 1861 censuses, and came up with 2, who were both traceable (Ruth Prescott nee Muncaster b 1818 Braddan, and Ruth Britton nee Gelling b 1815 Braddan ) but not your Ruth.