Surnames only started in medieval times and spellings only started to be standardised after the invention of printing and in some cases current spellings (both surnames and place names) may be only a hundred years old. As a result any analysis of the origins of a surname must involve a large degree of uncertainty.
Some surnames came into existance because someone was named after the place where they came from. It is therefore possible possible that your surname "Betlow" refers back to someone who lived at a place called "Betlow."
The earliest known reference to the Hertfordshire village/manor of "Betelawe"was in 1203 with other 13th century references being to "Betelowe," " Betelaue" and "Bettelawe." The village was abandoned in the Middle Ages (possibly due to the Black Death) and for many centuries all that remained of the manor was a single farm.
|The Moat at Moat Farm, Marsworth, undoubtedly dug to drain the central area for the Farm|