Please update. Have you been able to get PRTD? How long did it take to process the application?
The reply I received from the Visa Office suggested that I include my flight itinerary and a covering letter explaining my circumstances.
My plan is to prepare everything in advance, however, I cannot make an adjustment via London to drop off my passport but I could make it to London to pick it up once I had sent it in and it has all been processed.
After consulting all my options, I think providing the PRTD isn't massively delayed in returning to me then I should be able to make a sensible judgement about amending (for a fee) my return date home to Vancouver. All indications are that it shouldn't be a complicated application or that it shouldn't be delayed too much, I'd simply be at the mercy of the Visa Officer in terms of how fast it can be processed and hope the courier isn't too long in returning it to me.