There is actually DfT speed limit guidance that says they would welcome applications from local authorities for rural 40 mph zones, particularly in Areas of Outstanding Natural Beauty. To the best of my knowledge it hasn’t been taken up anywhere. (There have been some interesting projects in Scotland, though, where a few authorities have floated Quiet Lanes with reduced limits.)
I remember when they introduced the 40mph in New Forest National Park (mid 1980s from memory, as I moved away from area not long after), aim was to cut deaths of wild animals. Apart from some villages with 30mph, had previously been national limits.
It took about year to put and signs and paint the oval speed limit road markings. About 18 months later annual statistics for animal deaths and serious accidents were released for year and they had gone up compared to year before started introducing the 40mph
What seemed to happen is previously people had driven appropriate to road conditions, perhaps 60mph on open straight heathland sections, but just 25mph on narrow difficult to see bends. Basically old fashioned driving to road condition. But the new limits caused some to potter along at constant 30- 35mph, and lots more dangerous overtaking on the previously faster bits became common.
Ever since I have been against blanket speed limits where same limit is applied for simplicity rather than limits being set by road standards / Conditions.
Luckily where I now live in West Wiltshire, they set by conditions, many B and rural roads have mix of 30, 40, 50, national (60), and plenty of places where you can see all 4 within a mile as road transitions from town, to few houses, to rural road, to improved open section.