The speed limit was set at a time when cars were pretty ****. Yes, the law says 70mph, that certainly doesn't mean that going above that is overly dangerous. Speed limits need a massive overhaul in general, cars are safer and stop a lot faster than they ever used too.
You are getting far too bent out of shape over this. Underlying issues with your hatred of speeders.
Acceptable speed and driving perceptions vary drastically between people. I was going out Sat, following a red astra, constantly tapping breaks on, doing 50mph in a 60, a pain to follow. We came around a bend and I overtook. I knew nothing was coming as I had a clear view of the road ahead before we even got to the bend do to us approaching down a hill. Pulled in in plenty of time, no issues, my wife didn't even say anything. Go to the meal and it turned out it was the other 2 couples we were going out with and one told me I was driving like a C Bomb... we had rather a little falling out over it because in my eyes (and everyone else's) it was fine, but to her i may as well have curled one off on the grave of her grandmother.