Way in advance! You will have the flexibility of having 12 months from the moment you will get your POE or approval letter, so it’s always better to secure it sooner rather than later. It is a general rule that you should secure your new 12 months working holiday permit at least a few months before your current work permit expires.
While you will still need to exit Canada in order to activate your new work permit, having it ready will drastically reduce the logistics of this process, and will enable you to communicate your movements with your employer, securing your work, and therefore your future. Thankfully, you do not need to return to the UK in order to activate your new IEC work permit, so this can simply be viewed as an opportunity for a quick holiday!