If central England were to disappear, you could probably recreate it from this book. That's how gigantically detailed and realistic this book's setting is. No, let's rephrase our first sentence: You could recreate central England from this book, if you only wanted to recreate a few fields, some small rivers, and a farm. Since these are bunnies and not international bankers, most of the action takes place in a rural area that doesn't actually cover a whole lot of ground.
The important thing here, is that the setting is incredibly accurate. If you have trouble picturing any locations, you can check out the "Best of the Web," because several superfans have gone over this landscape and taken pictures. (If you ever go to England, there's probably a walking tour you can take to see the sights.) It's so realistic that Richard Adams points out which geographical survey maps could be used to see this setting.
In his introduction, Adams also notes that this setting is one he's very familiar with (if you had any doubts): "When I was a little boy I often walked on the downs with my father, who used to point out the birds and wild flowers; and thus began my lifelong love of natural history." So the fact that this book spends so much time on the plants and the natural setting isn't an accident. "Setting" to Adams means "nature," as far as bunnies are concerned.
Or rather, let's say that this super-realistic setting of England emphasizes the conflict between the natural and the human. Sure, there's some safe spaces for the rabbits (like Watership Down), but the rest of the setting shows signs of humanity's meddling—railroad tracks, a road, a farm, etc. In other words, if you're in England, you could probably walk outside your door and see this conflict played out in the fields right outside of town.
Or let's put it this way: maybe Richard Adams spends so long making the setting realistic because this book makes us ask how we're going to treat the wild(ish) spaces and nature right outside our door. If you can read this book and then go exterminate a warren to build an apartment building, you might have missed the point entirely.
Most of the action in the book takes place over less than a year. The story opens in May (1.2) and ends in autumn (50.58). But in what year? We don't know and the rabbits don't care. The narrator never tells us the year, which (a) probably makes the story seem a little more current (although, seriously, if we were writing this today, humans wouldn't be smoking so much); and (b) reminds us that the story isn't really interested in human life that measures the years.
Is there a war on? Who is president or prime minister? What is Nicki Minaj up to? The rabbits couldn't ask any of those questions, and since the book is primarily told from their point of view, we can't ask those questions either.