The situation has got progressively worse over the years since Covid. My eldest was 17 in March 2020, the issues for him were around sitting the theory because of social distancing but once he passed that, we managed to get a test, and a cancellation when he failed, fairly quickly at our preferred centre.
DD was 17 in 2022, getting a theory was OK by then but a test was a lot harder but we used the Testi app and managed to get one.
DS2 was 17 in 2025, bloody nightmare. Again, theory OK and he passed a week after his birthday, we could not get a slot for a test for weeks and when we did, it was in rush hour, in the middle of winter, at the busiest test centre in the city but that was OK because judging by DD's experience we thought we'd be able to change it. No chance. Nothing on Testi, nothing on the website. So he ended up sitting it anyway and luckily passed.
It is a combination of factors. The government not investing in the IT fixes to stop people/resellers booking miultiple slots. It would be so easy to code the site so you need a theory test pass certificate number to book a slot, and you can only book one slot with that number. No allowing instructors or other people to book slots to resell. I think they have started to address this but it is far from resolved. Also the salary thing, qualified people would far prefer to be self-employed as driving instructors rather than employed as testers. Raising their salaries would mean paying more for a test but that's probably better than paying out for so many more lessons while you wait.