So, I'm back from my visit. I tried coming back along the route Scenic had plotted ... and I'm now going to uninstall the app. I'm glad I didn't pay for it!
The route, as expected, tested my nerves. Some of the single-track road was the type where a car would hit their wing mirrors on hedges on both sides at the same time, with a thick line of gravel down the middle of the road, passing places distinctly off-road. But, it was definitely curvy!
However, what made the app unusable was the directions. Where roads bend to the right and it wants you to turn off to go straight ahead, it says nothing. Then, it sometimes tells you to turn when you don't need to, it's just a bend in the road with no junction. It kept calling mini-roundabouts T-junctions, and didn't quite get those right. There was a definite need to keep one eye on the screen, which isn't right. On top of that, it calls out directions in feet not yards, and the voice is a bit abrupt. I pulled over and switched to Waze to get me home, which was some relief, particularly as it didn't take me back to the M25.
Anyhoo, if anyone is considering it, I wouldn't recommend Scenic. Onward!