Political ideology prevents the NHS from charging for anything themselves.
A lot of people would pay for more convenient appointments, shorter waiting times, "nicer" hospital room, more convenient place of appointments, etc., but the political dogma won't allow for anyone to have "better" service, so everyone has to put up with the same (often crap) service.
Take NHS hearing aids. You're basically given what you're given, no choices, no option under the NHS for enhanced aids, such as bluetooth, etc. (yes I know some trusts now offer Bluetooth ones but most don't). So if you want something better, you go private and the private provider makes a shedload of profit. Why can't the NHS provide a "chargeable" option for a better hearing aid and then they could benefit from the charge and profit??
Same with standing/upright MRI scanners. Not available under the NHS, so you have to suffer the horizontal tube which a lot of people can't tolerate, and some can tolerate with sedation. If you want an upright (open) one, it's basically £1,000 privately. Again, why doesn't the NHS offer a "paid for" upgrade to the standing one for people who struggle with the horizontal/closed one.
There's so many ways that the NHS could actually make money by offering enhanced options/services to their own patients, but politics makes it impossible so those people will pay private providers instead. Makes no sense unless you're a political ideologist who wants everyone equal!
We have similar in dental and optician services (privatised) where you can pay fixed amounts (or zero if eligible) for the bog standard treatments, but can pay extra if you want something better, i.e. white filling instead of grey or a higher spec of glasses frames. But hospitals and other services under direct NHS control aren't allowed to offer that kind of choice.