Where does the term "Flashlight" come from?
The Century Dictionary and Cyclopedia, published between 1889-1914, has two definitions for flash-light:
n. A light so arranged as to emit sudden brilliant gleams, lasting but a short time: used for military signals and in lighthouses. See lighthouse.
n. flash-light A preparation emitting when ignited a sudden and very brilliant light, used in taking instantaneous photographs at night or in a room insufficiently lighted by natural light, etc. It usually consists chiefly of a magnesium powder, sometimes in combination with guncotton.
So the word flashlight precedes the invention of the device we most commonly call a flashlight today, which Wordnet 3.0 defines as "a small portable battery-powered electric lamp".
This helps us to establish that the earliest uses of the word were connected to lighting sources characterized by brief duration. When coupled with my personal experience with antique batteries and short-lived carbon filaments, it goes a long way towards proving that the explanation offered by The Flashlight Museum on wordcraft.net is likely to be correct:
The first trustworthy lighting device was the flashlight, invented about 1896. Portable electric lights were called "flash lights" since they would not give a long steady stream of light. The carbon filament bulbs were inefficient and the batteries were weak, allowing the user to flash the light on for only a few seconds, then release the contact. Very early lights did not have an on/off switch, just a ring or tab that would push against a button or band of metal.
Regarding why Brits prefer the word torch, one may as well ask why they prefer to call the trunk of an automobile a "boot", the hood a "bonnet", an elevator a "lift", French fries "chips", potato chips "crisps", cookies "biscuits", and biscuits "scones". I am sure the 3000 miles of ocean separating them from the U.S.A. has something to do with it.