Missed appointments don't cost the NHS money because GPs are paid for the number of patients they have on their books, whether they go to the doctor or not.
What they do cost is inconvenience to other patients who have to wait longer for appointments. It's currently a 3-week wait at my GP practice, and I know that's a lot better than many.
It's actually been shown that notices in waiting rooms showing the number of missed appointments don't actually reduce the number of DNAs and sometimes increase them. Text reminders or phone reminders are much more effective.
Because of the phenomenon of social reinforcement, when people see these notices, although the sort of people who wouldn't miss an appointment are shocked, people with more chaotic lives who are more likely to miss, get the subliminal message that because so many other people miss their appointments, it doesn't matter if they miss them too.
I don't think it helps that often these days practices are situated in weird places. Our practice used to be next to the library in the middle of the village, near the shops and the bus stop.
Then it moved to an industrial estate, not on a bus route and you can only get there by car or taxi and for many people it's too far to walk from the middle of the village where the public transport is.