First and foremost, the best advice is to stay off such substances permanently and forever and only to drink alcohol in moderation.
I completely agree that you should go and get a Class1 medical before anything else; that would apply even if illegal substances weren't an issue. So the CAA isn't going to raise an eyebrow if you get the medical done first because many others do just that anyway.
Never admit to anything on questionnaires for a medical, apart from "normal" childhood illnesses like mumps. Tucked into the list of ailments on the tick-box form when I did my first medical for the military many years ago, are innocent-looking things like persistent headaches/migraines, bed-wetting, glandular fever and asthma. These conditions are a definite NO for the military; I'm not sure about the CAA.
The advice I was given by an uncle who flew Hurricanes, Vampires, Harvards and Dakotas was "Don't tell the blighters anything that they can't find out for themselves". Well, none of the "fail if you admit" items applied to me fortunately, but you can make of his advice what you will.
Too bad if anything undesirable is already on your medical records, in which case the advice in my second paragraph still applies.