My guess would be that William Spence came from somewhere else as there are no burials for SPENCE(R) before an infant Martha in 1809 (see
www.lawsons.ca) and of those burials that indicate a date of death the earliest approximate birth date is 1779 (Margaret Spence buried 1846 aged 67). The earliest legitimate birth entries on the IGI are William and Jane's children.
Pretty much the same thing with BEEDON - earliest burial is Anne (no age given) 1790 and the earliest that provides an age indicated an approximate birth date of 1779 (Edward Beedon buried 1819 aged 40 - probably son of Patrick and Martha). The earliest legitimate birth entries on the IGI are the children of Patrick Beedon and Martha Munds/Muns/Munch. No marriage entry exists for Patrick and Martha though both are in the burials index (Martha bd 1799, Patrick bd 1822). Jane is not listed amongst their children but, if she is their daughter, perhaps she was born before the family arrived in IOM or perhaps she is Patrick's little sister or cousin etc.
Perhaps their families both came out when William and Jane were children, but I think it likely that neither was born there. You would expect them to be on the IGI (most of the IOM is pretty well covered) otherwise.
I would suggest you check out all the entries for Beedon and Spence on the Island - IGI online, Lawsons.ca, also Manx Notebook (
http://www.isle-of-man.com/manxnotebook/) and try to map out both families. Also, do a search on the IOM genealogy message board (
http://www.isle-of-man.com/cgi-bin/interests/genealogy/bulletin/index.pl). They are both uncommon enough for this to be manageable and will help.
Good luck,
Ruth