To be clear, I'm asking about the idea of space travel being impossible. I'm not interested in asking about astral projection right now, though I'm sure the history of that concept is interesting in its own right.
The crucial and delicate matter of getting the characters off the earth must be very carefully managed. Indeed, it probably forms the greatest single problem of the story. The departure must be plausibly accounted for and impressively described. If the period is not prehistoric, it is better to have the means of departure a secret invention. The characters must react to this invention with a proper sense of utter, almost paralysing wonder, avoiding the cheap fictional tendency of having such things half taken for granted. To avoid errors in complex problems of physics, it is well not to attempt too much detail in describing the invention.
While it is difficult to claim the very first instance of astral projection for interplanetary travel, it is important to recognize two distinct branches of early science fiction/science fantasy in this regard. On the "harder" end of the scale (relatively speaking), you have works like Jules Verne's De la Terre à la Lune, trajet direct en 97 heures 20 minutes (1865) and H. G. Wells' The War of the Worlds (1898), which posit some form of spacecraft to cover the vast distances involved. On the "softer" end of the scale, you have works like Edgar Rice Burroughs' A Princess of Mars (1912) and E. R. Eddison's The Worm Ouroboros (1922), where transportation is involved either through a form of astral projection or teleportation.
The ultimate origins of this kind of travel are a bit vague, although a good case in science fiction could be made for Mark Twain's A Connecticut Yankee in King Arthur's Court (1889) as providing the essential elements:
“You know about transmigration of souls; do you know about transposition of epochs—and bodies?”
Where the protagonist gets hit in the head with a crowbar and wakes up under a tree thousands of miles and a thousand years away. The use of teleportation or astral projection - the really amount to the same thing, from a strictly narrative standpoint - allows the character to emerge directly on an alien world with no intervening travel. It skips over the hard physics of space travel, atmospheric exit and re-entry, etc. Not that these were necessarily a problem for early sci-fi authors, but for those that didn't want to dicker about those details, it was already a well-established trope by the time Tolkien was considering it in the 1940s.
Was that a common notion in 1940s Britain and if not why would he hold it?
This was a pretty common notion among anyone that looked at the matter seriously: the distances involved were vast, and the technology of the period just wasn't there yet. You had plenty of Buck Rogers fans that recognized space ships and interplanetary travel in fiction, and rocketry excited a lot of imaginations for the possibilities of space travel, but advancements in physics and astronomy were making it more and more unlikely that nearby planets were habitable and the risks and obstacles involved were becoming better and better known.
I also talk a little bit about the development of the idea of space ships in It's generally taken for granted in sci-fi that a space military would basically be naval, with cruisers and frigates in fleets led by captains and admirals. Given that there has never been a space military, how did this assumption develop?