99 Flake

A 99 was a frozen dessert that was superior in value to a choc-ice. When a Cyberman snuck up on the Tenth Doctor and said, "You will be upgraded," the Doctor — lost in thought about the virtues of choc-ices — initially believed that he was eligible for a 99 at the same price as a choc-ice. (COMIC: The Power of the Cybermen)

Behind the scenes
The Power of the Cybermen never uses the term "99 Flake", but such is what "a 99" means in modern British vernacular. Contextually, too, it makes sense that the Doctor means a 99 Flake, since a 99 Flake would indeed be an "upgrade" from a choc-ice.