Agree, Callistemon For him, it is a normal expression, just as mathematicians, scientists and social workers have a lot of normal expressions that they use in everyday speech without doing it to show off. Falling over, to my mathematician father, was always "going base over apex" and talking of how much dosh an acquaintance had accumulated over a lifetime of high salaries he referred to "the total under the curve". We knew what he meant.
However, Boris Johnson was speaking to the nation, and Scylla and Charybdis are not normal enough in the general population for him to use in that situation.