I can only speak from my own experience of myself and others in Private Practice over a period of 30 years as an NHS Consultant.
Of cause, like other trades, professions,public sector workers and private sector workers, there is always the opportunity for "little fiddles", no big deal, but "perks" all the same............and the medical profession is no different.
To indulge in private practice, one has to take a drop on salary, but continue to give the same service............what union member would accept that?
You do have a point in that many private hospitals do not have the same facilities as the NHS hospitals, but this is improving and during the last 10years of my "time" consulting in 3 private hospitals , the facilities were adequate. This is the same in NHS hospitals, some are excellent, some adequate and some, really sub standard.
Private Practice is hard work.....you build it up by giving a good service in the NHS , so patients in the NHS benefit.
I don't think that i have given the answer that you wanted, but please come back to me with any queries.