Why are "flashlights" known in the UK as "torches"; and, to be fair, why are "torches" known in the US as "flashlights"?

In Britain and all English speaking countries outside North America, torch usually means a flashlight, and a torch which uses fire as its light-source is often called a "burning torch" to distinguish.
~Wikipedia
The term flashlight is used mainly in the United States and Canada. In other English-speaking countries, the most common term is torch or electric torch.
~Wikipedia

In the UK, they were in fact 'electric torches' at first, and if I remember correctly, are called that in Arthur Ransome's Swallows and Amazons books (from around the 1930s). As on the whole we had given up using large smoky things for light, the 'electric' bit got dropped. Torch by itself normally means the electric thing, but does still retain its earlier meaning in contexts like the Kendal Torchlight Carnival, where the torches are large and on fire. (Well worth going to if you get a chance - they turn off all the street lights and the procession is lit by the torches. It goes for 2.5 miles and feels a lot longer when you are in it...) http://www.kendaltorchlightcarnival.co.uk/
Flashlight is occasionally used in the UK, but my spellchecker objects to it (and to 'spellchecker' too). Not all torches flash - having a flash button was considered a great bonus because you could send messages in morse code. Or could have, if both of you had actually learned the code. Possibly North American ones were more fitted with flash buttons than ours were.

In the US, torches are used by welders.

https://www.totaltools.com.au/welding/welding-accessories/welding-torches

Same here - but you can't really use them for lighting....

Not on a camping trip, but if there is nothing else around during the end of the world.

Link where I found information.

https://en.wikipedia.org/wiki/Flashlight