Surely motorways were designed before car drivers could even think about 100mph? And anyway, if it's not advertised how do you know about it?
To be quite honest, I've not seen the 100mph figure in any official publications, however it is a number that is used in a lot of road safety websites. And motorway corners certainly are sufficiently sweeping to allow 100mph with ease.
But, as I understand it, when the motorways were originally built, the plan was to have no speed limits. As you say, most car drivers could only dream about 100mph back then. 100mph seemed a reasonable design speed to accommodate future advances in car design.
Then, legend has it, someone drove an AC Cobra at 180+ up the M1, so they introduced the 70mph limit. Although, in reality it was due to too many fatal accidents occurring when unsuitable vehicles failed at high-speed (tyre blows, engine blows, careering into oncoming traffic due to no barriers)
Having driven on motorways in many other countries, it's obvious that our motorway design speeds are higher than in other countries; longer slip-roads, wider lanes, less severe corners, better crash barriers etc.
Most worrying in my experience is the Italian Autostrada. It had no speed limit until a few years ago, but has sliproads so short and steep you're lucky to hit 50mph at the top in a typical modern car, NO HARD SHOULDER in many places and corners so tight that they need to lower the speed limit around them!