Unfortunately, much as I would like to be giving people better news on this page, despite cannabis being legal on prescription for a number of years now, it does happen occasionally that even legally prescribed consumers get stopped from time to time as unfortunately, apart from a startling number of officers being unaware of the law change, my understanding is that road traffic laws haven't been updated accordingly, muddying the waters further. Your prescription is your legal defence, but it's still best to be as aware of your rights as you can be in case issues arise. This is about the most up to date set of resources you'll find and I'll endeavour to keep it that way, but if you're a driver I strongly you recommend you read them just in case. One is actually guidance aimed at assisting police by clarifying the legal actuality. Most were written by or involved Guy Coxall from Seed Our Future, and you'll struggle to find someone who knows more on the subject.
As they are all on one specific topic, I've divided them (each section alphabetically by author) firstly Government Publications and secondly, Published Guidance pertinent to Cannabis and Driving as I figured the second probably wouldn't make much sense if you hadn't read the first.
Government Publications and Law Pertinent to Cannabis and Driving
Department for Transport (UK) (updated August 2017) Changes to Drug Driving Limits. Gov.uk. Crown Copyright. Available online at: .https://www.gov.uk/government/collections/drug-driving#table-of-drugs-and-limits