You're quite correct newcomer, but you refer to a totally different instruction, where the controller has specifically asked you to manipulate the "C" facility of your transponder. The "A" prefix you mention previously does not mean "mode A" on your transponder, it refers to something else of which the definition escapes me right now without the books handy. You won't find it in CAP413, it is more likely to be found in MATS. Perhaps someone else would elaborate with a definition.
In the meantime, if it helps, think of the instruction "squawk A1234" as "squawk A1234 with mode Charlie".
Controllers rightly expect that good airmanship will dictate squawking mode C at all times and should rarely require instruction to the pilot.