I think you're making too much of this. The CAA medical form asks some quite specific questions; it does not ask about one panic attack caused by one drug a very long time ago. Lots of people have had occasional panic attacks. Lots of people have taken drugs - whether legally or illegally is hardly relevant here as we're talking about ONE incident.
The people who answer the phone at the CAA are notorious for not being experts on what you want to know. Of course whoever answered would tell you to go to your GP etc if he/she didn't know. From their point of view, that can hardly be wrong, can it?
If you're actually worried about the effects of this incident, talk to your own GP. Otherwise, answer the questions on the CAA medical form honestly and completely, but don't add anything else. Because from what I've seen,although I can't imagine they would stop you flying because of it, such things seem to sometimes delay the issue of a medical...while they think about it! When I went for my first medical, the AME told me to fill in the form, no more and no less. And they've certainly never contacted my GP.
It might be worth remembering that some people have had occasional panic attacks and no-one knows about it, and amphetamines used to be prescribed for various conditions - I got a very low dose at the age of 16 to help me lose weight (please don't start a discussion on the wisdom or otherwise of this; I stopped taking them after a week as I couldn't sleep!). You are making too big a deal about this. Some of us would even have forgotten it occurred...11 years ago....jeez, I can't remember what happened 11 days ago sometimes.
So go and get your medical and stop worrying.