Dick, who embodies the spirit of the British empire, immediately wants to bring technology to their lives, but his first effort at steam power goes badly, and his attempt to introduce the wheel is met with horror. There is good reason for the horror.
The medieval-esque peasants aren't the only class of people around; there is also a ruling priestly class, who practice mathematics to excess and hoard technology to themselves. They are clearly Bad. For instance, as well as looking Bad--they have bald shaven heads and glower nastily, they are the only ones who get to use the wheel, and they do, as a means of killing people who dissent or disobey their fanatical rule. They have used their own rather marvelous technology to bring people from the past to add to their store of knowledge. Attempt #1 brought them a Neanderthal, which wasn't useful, Attempt #2 was a winner, resulting in a 22nd century engineer (who wants to get back to his family), and Attempt #3 another dude-the two boys. But even though the boys aren't any use to them, they aren't going waste time sending them back.
There is also a Resistance of people fed up with being killed on wheels, featuring a remarkable girl who can do everything physical (archery, riding, hunting etc.) better than the boys (Dick is very taken with her), and her brother, described over and over again as "the crippled boy" which grated more than a little. Adventures and perils ensue as the boys and their allies break into the Mathematicians citadel, bring down their regime, and get home.... except for Dick, who chooses to stay with the Atalanta of the resistance (one gets the sense that he can't leave until he is better at archery, riding, hunting, etc. than she is....)
This is a book best read by 1950s children who haven't read all the better books still to come. The plot, though fine, is not quite well developed and nuanced enough to make this a great book, and the story is slowed by lots and lots and lots of description (so much description it was hard to actually picture anything due to my mind's eye glazing over...). There isn't much to speak of in the way of characterization, or finely drawn emotional peaks and valleys. I doubt I will ever want to reread it.
However, this is an interesting one for us serious readers of juvenile time travel. E. Nesbit's The Story of the Amulet is the first time travel book in juvenile fiction written in English to take children into the future, but that was just one episode of many. The Future Took Us seems to be the earliest book published in English that makes this sort of time travel the plot of the whole book.* This particular twist on time travel hasn't gotten much attention since (in my time travel book list, there are only ten that involve travelling to what is the future from the main character's point of view). Bits are reminiscent of H.G. Wells, and Severn adds very little in the way of wild original imagination. The time travel ends up reading more like a portal fantasy to an alternate England than travel to a future one, and I wonder if Severn had just read the Narnia books when the idea for this story came to him.
This is the fourth book by David Severn I've read. Several did get published here in the US, but don't seem to have caught on. Though this one was disappointing, I will still be looking out for his other books, especially Dream Gold, which Kirkus rather liked and which is still, according to Worldcat, on the shelves of the Colorado City, TX public library (one of his books even got a Kirkus star). I can see why he didn't make it into the cannon of great mid 20th century English books that are still in print, but the two books of his I've read that aren't The Future Took Us and Drumbeats (another time travel story I didn't care for) come quite close to being ones I really liked a lot.
*feel free to point out ones I missed!