Free at point of delivery means to most people it is viewed as being free, and is treated as such. If a deposit was paid on booking an appointment and refunded on attendance (or cancellation in a given time frame) that might also help focus minds.
On one shift I was sworn at in (I think) 10 or so languages mainly for the crime of not giving antibiotics for viral infections and of course as a Locum to make a fuss might mean I do not get retained again.
The worst offenders are unlikely to be managed by this - most will have no money and do not mind using various GPs or indeed A&Es. But this could certainly help for the majority.