The UK came relatively late to Motorways, and benefitted from the experience in other countries that lengthy straight sections tended to have a higher accident rate, and thus a series of gentle curves is part of the design where practical.
ISTR that motorways can't/couldn't/don't have perfectly straight stretches of more than a mile. As indicated by WHBM, this was to keep us awake! At least, that's according to WSBIAPTM.*
What about Oz? They seem to have straight roads that go on for ever.....
[Edited to add:]. Dr Google says Oz's longest stretch is 91 miles!!!
* What Some Bloke In A Pub Told Me.