Except for the age of the protagonist, everything matches the beginning of the Sword of the Spirits trilogy, by John Christopher: The Prince in Waiting, Beyond the Burning Lands, and The Sword of the Spirits. They were first released in the early 1970s. They have apparently been released as a single-volume omnibus edition (as The Prince in Waiting Trilogy), which would be the right length—and also as a boxed set.
The stories are set in the South of England, mostly in Hampshire, in a post-apocalyptic future where, due to a worldwide ecological catastrophe, life has reverted to a militaristic, medieval setting of walled cities and perpetual warfare. Christians are a despised minority, as spiritual matters are in the hands of a priestly class of monastic "Seers" who interpret the will of the "Spirits". There are signs of the past existence of the modern world in the ruins of great cities and "high roads" which dot the harsh landscape, but the Seers have made the technology of the "ancients" anathema, and anyone dabbling in "Science" is immediately put to death.
The catastrophe has also resulted in a greatly increased number of birth defects. People with dwarfism constitute a separate caste to "true" men and fulfill the mythical role of forging weaponry, along with other metal work. People called "polymufs", who have other disfiguring mutations, are the lowest caste, as menial workers and wage slaves, regardless of whether they are actually physically or mentally disabled.
The protagonist is Luke, a teenager whose father becomes prince of Winchester, one of the walled cities that dominate post-apocalyptic England. He gets involved in the cutthroat politics of the city and eventually learns that the Seers, the religious leaders who enforce the prohibition against technology, are actually preserving and using technology themselves, hoping to eventually guide people back out of their dark age.
In the second book, Luke literally travels over a pathway of cooled lava. A range of active volcanoes has emerged separating England from Wales (the titular Burning Lands), but a pass is found across them, and the later books focus on the interactions between the English and the Welsh, who have very different civilizations. (True to the image on the cover shown above, Stonehenge features prominently in the story, and Luke does eventually arm himself with a Sten gun.)
For other questions where this series as come up, see:
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