Hi, John, and welcome to TP. I am very sorry to hear about the situation with your father and you must be very stressed/upset by it.
You will get other responses here, and if you like, you can also have a search as this is a topic that comes up often.
I think that you might want to communicate your concerns to the eye doctor ahead of the appointment (letter, fax, hard copy handed unobstrusively to the receptionist when your dad isn't looking). I would avoid any mention of dementia in front of your father, as this will likely only lead to upset.
The hard truth is that, if your dad should not be driving for whatever reason (and we do believe you about the undiagnosed dementia), then he can demand all he likes, but you don't have to comply. In fact, if driving would put him and others at risk, then I would definitely not give him access to his car. (This is the old "can doesn't mean should" scenario.)
Many, if not all, of us here on TP have been in the very uncomfortable situation of having to walk the fine line between trying to give the PWD (person with dementia) what they want, while still making sure everyone is kept safe. This is never easy or pleasant and it's very difficult when we know "safe" is the answer but for whatever reason, it feels wrong/hard/disloyal/upsetting.
One thing I have learned is that in many situations like this, even if the person doesn't have dementia, that you should carefully consider the messenger as well as the message. The adult child is usually not the best choice to deliver the message. In other words, blame someone else: the DVLA, the doctor, the car itself (which can be "broken" for a long time), the insurance company, the mechanic who didn't fix it properly, et cetera.
I am in the States, but believe in the UK, you can have a physician contact the DVLA for a driving assessment? If the eye doctor will not get on board, I'd enlist the GP. Again, keeping the onus/stigma off you as much as possible is a good idea here and in other situations.
I can tell you I went through the driving issue with my mother, did not remove her car soon enough, and she continued to drive and had two accidents that I know of, mercifully only slight injuries to herself and none to others. Although she hasn't driven for over a year now I am still haunted by what might have happened. I would not have been able to live with myself had she injured or killed someone.
It is very difficult and I am sorry about the situation. TP is a great resource and I hope you will find help and support here.
Best wishes to you.