I know of at least one St. Beghs marriage that wasn't registered (apparently). I told a distant cousin in Ontario to contact them on their website, and she was sent info, including the marriage (17 Dec 1891) of Dominic Burgoyne and Elizabeth Ann Guthrie, but when I looked at Freebmd I could find no record of it. I once wrote there myself, and received info which included another marriage that seemingly never took place.
An ancestor of mine, Robert Cowan/Cowin, obviously did a runner around 1864/5, since his name was not on the youngest daughter's birth cert (there was also no baptism). There was no further trace of him. A man with the same name and occupation was living in Alabama in 1870, then Oklahoma, but the censuses say he was born in England, instead of the IOM - although I know the IOM can be classed as England in American censuses. His wife remarried to Christopher Reay in 1873.