This might be helpful in explaining why "excess" or "shots for people who don't turn up" are being used as they are. It's from the US, but the methods of delivery won't be different.
slate.com/technology/2020/12/covid-19-vaccine-nursing-homes-pharmacist.html
Specfically - "You have to let the Pfizer vaccine come to room temperature before you can do the reconstitution process. Once you’ve reconstituted the vaccine in the vial, it’s stable for a little bit of time, but once you draw up the vaccine in the syringes, it has to be given immediately."
and "After administering the vaccine, we observe all patients for 15 minutes to make sure they’re feeling OK. If patients are deemed to be at high risk for a reaction—meaning that they’ve had a previous anaphylactic reaction to a vaccine or other injectable medicine that caused them to seek medical treatment, such as an EpiPen—they are observed for 30 minutes."
If someone doesn't turn up for an appt, it could be that there is very little time to find someone else to take it, someone who could be found very quickly and who could spare up to half an hour or more away from their post. Even the article quoted above only suggests it's a handful of non-front line staff getting the jab.