John John 
After London [EPUB ebook] 

After some sudden and unspecified catastrophe has depopulated England, the countryside reverts to nature, and the few survivors to a quasi-medieval way of life. Beginning with a loving description of nature reclaiming England — fields becoming overrun by forest, domesticated animals running wild, roads and towns becoming overgrown, the hated London reverting to lake and poisonous swampland — the rest of the story is an adventure set many years later in the wild landscape.