If you'd read any research into the reasons why people miss appointments you'd know part of it is poor administration, part of it is the length of time it can take for the appointment to arrive so matters have resolved or been forgotten, part of it is issues like transport, childcare, time off work, and a lot of it is that people feel disrespected by the healthcare system and therefore feel no obligation in return.
Disrespected because the deliberate overbooking means they always wait for ages (which further adds to the idea that it makes no difference whether they formally cancel an appointment or not), because of difficulties getting an appointment on a timely basis, because of poor provision, because of turning up to receive test results and finding they're not ready or haven't been chased, etc etc etc...
If the NHS wants to change the levels of missed appointments then it needs to address its own responsibility for the problem too instead of making it out to be about lazy/selfish/ungrateful time wasters. To a significant degree, the problem of missed appointments is of the NHS's own making.
I always cancel appointments I can't attend. Does my hospital have the competence to record that I cancelled it? Nope. I've had to contact them repeatedly just to try and make sure my cancellation is processed, because I know from past experience they are hopeless. It's fucking ridiculous and wasteful. If they don't record that I called them three times to cancel an appointment, they'll presumably record it instead as a no-show. Incompetence doesn't leave that much of a paper trail in such a scenario.
Calculations of the supposed cost of missed appointments is something of a nonsense too.