This is almost certainly "The Alien Earth" by Scottish author Michael Elder (1931-2004). Let's see if I remember correctly: Astronaut in distant future has accident in space, is thrown back in time and lands his capsule on Earth (what the reader soon realizes), but in the distant past. First act: Hadrian's Wall, interaction with Roman soldiers and Picts; no help possible for damaged spaceship, swallows pill, wakes up in more or less present time (from a 1950s point of view). Scenario: England, a little bit dystopian, WW3 looms on the horizon. Spaceship buried beneath the ground, in some unused lot. Befriends lonesome spinster, realizes that technological level is still too low to repair spaceship. Swallows pill as air-raid sirens herald nuclear holocaust. Third act: wakes up in distant future, in underground society who worships him and his spaceship. Realizes that people speak his language. Falls in love with beautiful priest lady, finally coming home. End of story as correctly remembered by you.
Cheers from Innsbruck, Austria