On the Royal Ascot, Manton writes:
Not that I've ever been, but ... from the pictures it seems that formal morning dress is expected, cutaway coat, top hat, etc. I suppose a formal ascot -- worn with a wing collar and pearl stickpin might be OK, if you want to look like the little plutocrat from Monopoly. But not an ascot with an open collared shirt.
My brother, who is now at Oxford, tells me that they are uncommon, though not unheard of, during the event (I think that he likes the costume). Interestingly enough, the same type of guys who would have worn an ascot with an open collared shirt, or even a tail-less dinner jacket into polite society, are today probably the guys wearing hoodies under sports jackets and pairing straight ties with tuxedos (even... notch collared tuxedos), with the older generation looking on in horror. Just some food for thought.