All I can say - from experience of radiotherapy albeit salvage - is be prepared for plan B!
While the Churchill may be different, in my hospital the plan was evidently that the planning scan should be done at high resolution in the absolute optimal state of both bladder and rectum. That isn't easy to achieve, in my case they decided there was still too much in my rectum, I needed another mini-enema which inevitably was accompanied by urination and therefore another hour or two to get the badder back to their target level of content. It was a long and stressful day.
But as I understand it, the aim is to get a high resolution scan that allows them during treatment to adjust the dosing according to how the actual situation that day (determined each time by an initial low resolution scan) varies from that "optimum". The point being to target the cancer as best possible regardless of minor variations in location due to bladder or rectum fullness. (In practice, because bladder fullness was the biggest concern they did an ultrasound scan before I went on the machine, so I could readjust if necessary).
Good luck. The folk who operated the radiotherapy machine were amazing, dealing with people dealing with scary and embarrassing issues in a very empathic and supportive way.