The Cadbury website says that the reason behind the Flake being called a 99 has been "lost in the mists of time" although it also repeats an article from an old Cadbury works paper, which states the name came from the guard of the Italian king which consisted of 99 men and "subsequently anything really special or first class was known as 99.
A propos the subject of ice cream, something just popped into my mind the other day. When I was a kid cornets were for children only; adults buying ice cream would have an ice cream sandwich, i.e. ice cream between two wafers, very ungainly. You don't see those any more.
My mind may be wandering (age related ),but seem to remember as a child getting an ice cream cone with a flaky chocolate bar inserted into it. This was called a "Black Man ",but this name was discontinued some time in the fifties,for obvious reasons.
I was told a long time back that the length of the Flake was 99mm - but I think that is bogus and prefer some of the reasons offered in the link at the top.