For reserved englishmen, a slight inclination or nod of the head, no more than 10 degrees from the horizontal, and lasting less than 500 milliseconds, eyes front, was what I found when I looked it up in the "Englishman gentleman's handbook of motoring etiquette".
I also noticed that we are supposed to very noticeably avert our eyes whilst a lady climbs into any low-slung sportscar. Gosh, what a shame.
There is also a long section about correct behaviour whilst using public toilets in the course of a long drive. I won't go into that, and anyway, I would not want to look.
