Employer not making reasonable adjustments

I can sympathise. I work in a technical engineering department, and in my own employment can see that it wouldn't take much for my employer to get formal about attendance too.
There's a couple of things I always do. I always make sure that my Nurse, GP, or anyone else I have an appointment with know that I can't take time out during the middle of the working day. So I make appointments for first thing in a morning, or last thing in the afternoon. They are the less intrusive times of the day, and so far so good.
I always offer to make up my time. Sometimes it's expected, other times my boss will tell me that it doesn't matter. But I've found that it's better to make the offer than just to expect to get the time off. I have also used holiday days to cover for longer appointments.

If you are seen to be going above and beyond in order to compensate for the time you are taking off, then there's not a jot that they can do about disciplining you.

By law, they must allow you time off to attend medical appointments. They are not obliged to pay you for this though, although many do. You are covered under The Equality Act, although it is a bit woolly to be fair.
My only advice would be to record and document everything relating to this. Ask why you are facing disciplinary action, if it is purely for attending appts, you should be fine. The only potential problem is lots of time off because of poor self control of the condition. Although this may not be your fault, it can be argued by an employer that you are having excessive time off due to failure to manage your own condition. Hope things work out for you !
Mo