Nature Near London

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.