I think one of the (many) problems with NHS appointment booking systems is that they are developed by people who have too little in common with the intended users. They're developed by well-educated professionals in full possession of their faculties, who mostly mix with people of similar backgrounds. This applies to the systems for getting a GP appointment, choose-and-book for treatment, getting consultant referrals, and selecting the most appropriate service (Out-of-hours, urgent care, minor injuries, A+E, etc) when you develop a health issue when the GP surgery is shut.
All of these are perfectly simple to navigate if you're calm, not in pain, have good English literacy, you're not currently suffering with mental illness, you don't have any diseases that cause cognitive deterioration and you're not on medication that causes brain fog, and you're not pressed for time when reading the directions. Or in other words, you're in the best of health.
If people were in the best of health, they wouldn't be trying to get an appointment!
The bulk of people needing medical treatment themselves are going to be in pain, anxious, elderly, or struggling with mental health and so on. People booking on behalf of family members will be anxious and panicking. The end result is that people just go to A&E because that's the department they remember exists.
And let's not even get on to what percentage of staff in the care sector speak English as a second language or the percentage of native English speaking staff who have poor literacy. They need to be able to book appointments on behalf of service users and residents.