NHS informatics are notoriously crap. My insurance company knows more about my health than my doctor. My dog, (who is "chipped") has more information about his immunisations (in his neck) than I have.
In all honestly, you'd probably get away with hiding from your records for a while, but it's likely that eventually things would catch up with you.
Mental illness is no different than any other illness, people suffer from it, get treated, and generally recover.
I'd like to think that the aviation authorities share the same attitude....previous posts on here would suggest that you may have to jump through a few hurdles, but they are not insurmountable.
There's no guarantee of success, on a positive note, it would seem that your illness was some time ago. I suspect that the authorities would be interested in recent events/treatment.
As Radgirl notes, you owe it to yourself and your future passengers, to disclose. This may be at the risk of not gaining your ticket.
Please let us know how you get on.
