(Ooh! My first Kickstarter pledge!)
I know that a lot of these will have probably come up in some form in your brainstorming already, and some may already be being implemented for all I know. But here's a whole load of things I've not seen mention/confirmation of yet that came to mind when thinking of Victorian England/rural Britain that you could consider for inclusion now, or in a future update/DLC. I have used some subtle subliminal suggestion techniques to guide you towards the ideas that I feel add the most English countryside feel.
Wood Pigeon (especially the sound, also edible like pheasant)
Ducks (again, the sound plus edible)
Geese (maybe just flying high overhead occasionally in migration V's, with a bit of honking)
Deer (Suggested by others on this forum)
Foxes (also suggested by others here already. They could distract robo-dogs and The Riders, putting them off your trail)
Badgers (Also suggested by others here and on Kickstarter)
Swans (with some form of penalty for killing them, or a simple admonishment of "Oh sir, that simply is not cricket")
Black Dogs/Pad Foot/Black Shuck
(The legends that inspired the Hound of the Baskervilles that some others have suggested including. Basically a big black (ghost) dog, up to the size of a donkey, usually with glowing red eyes. The legends are of some sort of ghost, but if it were me, I'd make it a flesh-and-blood dog that appeared between midnight and dawn. Perhaps it could spawn and walk along the roads, attacking any player or npc who gets too close)
(a bit odd for a archipelago of small islands, but if the islands are big enough... Restricted to roads obviously)
(Best if you look up the stories if you don't know them already. A unique independent character who could appear on occasion, probably looking fairly inconspicuous, who would target players or npcs alike for an annoying whack to the back of the head before escaping. Alternatively, just his boots as rare loot to be used by the player for quick/comical get-aways)
(1-2 per island, non-hostile unless they see you bag a pheasant or duck. Give them a dead fox (tail?) for a reward)
(1-2 per island, non-hostile unless they see you harm livestock. They reward you if you bag a rabbit, fox or wood pigeon)
(agree a temporary ceasefire with your robotic pursuers to hunt a common foe)
(robotic ones, obviously)
Does the terrain generator produce woodland/copses? Not a deal breaker, but small areas of denser trees would add variety.
In villages - Cricket pitches & bowling greens
I listened to the background sound in your latest video, I heard crows, some sparrow-like tweeting, and some similar simple small-bird tweets. Apart from the wood pigeon and ducks mentioned above, some more complex bird calls would add a lot of countryside atmosphere - blackbird, robin, wren, chaffinch etc, and owls at night. It might not seem like much, but anyone who lives in the UK will have heard these and more, and it will add a huge amount of atmosphere. Google around for examples, just don't add collared dove - they only appeared in the last 30 years or so.
Fish in ponds & lakes for food, or just to get away from the wife & kids
Another item suggested by others in Kickstarter comments.
Apologies for length, but I felt it better to get it all out in the open.
That's an excellent suggestion.
Haha, yes, we should have thought of that.