I posted about this a while back ... DW had been sent an appointment as an NHS patient to a BUPA clinic, and the booking letter said that payment details had to be provided even for NHS patients.
Seems there is some point in writing to your MP ...
Thank you for contacting me some time ago with concerns that your wife was asked to provide payment details before she received NHS funded treatment.
As you know, I wrote to the Secretary of State for Health and Social Care to raise concerns on your behalf. I have recently received a response from Steve Barclay MP, the Minister of State for Health, a copy of which I have attached for your information. This response clearly states that your wife should never have been asked to provide payment details, and that the provider who did this was in violation of the NHS constitution.
If you would like me to raise a complaint with the CCG about this matter, then please do forward me a copy of the letter that she received, or the details of the provider.
So, it's not "standard", and should (and will be) be challenged.
I wonder how many people would have felt obliged to provide details
Bearing in mind this was a boilerplate letter, so sent to all patients.