I'm on an NHS waiting list that's around 3 months.
If I were to pay several thousands for the surgery using the same surgeon, same hospital etc that wait would be less than a month!
That's only one surgeon.
This is why the waiting lists are so long.
I'm sure they'd defend this with done kind of valid sounding argument but it would still come down to them wanting to squeeze month out of people and being allowed to do it.