I'm fully aware what prioritise
if you can means, but of you wanted to fudge the figures your simply defrost far more than you have for appointments scheduled. This could be done under false pretences - e.g. intent to have people phone their way down the JCVI list.
If unsuccessful grab whoever is walking past.
I'm not saying they are gaming the system but it'd be interesting to know how many excess people are getting vaccinated outside the JCVI list. I bet nobody is counting.
FWIW I also think it's better to inject someone than throw it away. I'm only pointing out that it's possible to game the system if your intent is to have a high headline figure while "prioritising the vulnerable".