I keep hearing this but surely if you presented your prescription in court the charges would be dropped? In my experience, magistrates are usually very reasonable. Police on the other hand...
Whether magistrates would look at the levels of THC in your blood and be sympathetic to an argument that you had consumed a long time ago (and thus were no longer under the influence) is another question. I tend to think magistrates would side with the law as written in most cases, if the law explicitly prohibits driving with any amount of THC detectable in blood tests.