Three of my usual evening fasting tests last year were above 30, which I double checked to make sure the reading was accurate. A 32.2, 32.3, and 30.5. They were not close together (a one-month gap, then a four-month gap) so I never went to A&E or sought urgent help. They also came as a big surprise as I was not feeling any symptoms from them.
I was obviously quite worried after the first one (which was pre-Covid) but all my attempts to request help in general from the surgery were ignored, despite me repeatedly pointing out those extreme readings. Then in summer the practice pharmacist needed to contact me about a prescription from a clinic, so I asked him for help and he told me such a high reading was impossible!
Mind, he then also told me that my average fasting levels being consistently around 13 in the morning and 15 in the evening did not imply anything about my overall level. Because my last HbA1c a year ago had been 7.2% / 55 mmol/L it was just a coincidence that I happened to have an exceptional spike just at the moments when I tested pretty much every day for the past six months. Apparently had I tested hourly they would have been much lower. Bizarre. And scary. I know he is not a doctor, but he is still allowed to give medical advice!
So in my case, what happens when your levels go high is not much. No one seems to care much and some people reveal staggering ineptitude. Otherwise, I never felt any different or seemed to suffer any ill effects. My foot check was good last December, and nothing new on my eye screening two weeks ago. And things are much better now thanks to a DSN who finally listened.