http://www.tricho-tech.co.uk/advantag.htm
"... Differentiates between one-off incidents and addictive behaviour ..."
So there you go. You're covered. You don't use canabis, but you've had some in your system once, recently - at a time when you know you're soon to be tested anyway - why would you do this for the first time when you know you're going to be tested? Tell them what has happened - excluding the bit about this baker-friend of yours being a special-friend who takes drugs regularly, because you might as well just hand your daughter over then and there if you admit you have drug users around regularly.
If you're not a regular user, and have a reason why you've got a sudden one-off occurance of the substance, it'll likely be over-looked.