Hi @Collwog Best advice in your situation is get in in writing. Create a paper trail that can be used as evidence in your favour.
Get everything in writing. But be as polite and accommodating as possible.
The Equalities act of 2010 prohibits punitive measures, victimisation and discrimination in the workplace.
Put a "special delivery" letter in to HR stating you were given permission from you're the company's representative (HR in your case) to take a break on the day if you needed it. Your employer hired you knowing you are diabetic; they are legally required to
"make reasonable adjustments" to accommodate your condition under the Equalities act of 2010.
However that being said you may find as a new employee you are under a 3 month probation period where you can be "let go" with out any explanation during this period. no need for verbal or written warnings during the probation period.
If they do use your Diabetes as an excuse to let you go
that is clear and provable discrimination and against the law you would have a case to take to a tribunal this will take months. Even if you do win there are limits in place for how much you could receive is you win,
it's not a lot a couple of hundred.
The reality is you are probably in a no win situation your are trying to keep a job you don't like, hopefully your boss should have had time to understand the situation better realise they are in the wrong and try to find a way to back down, that allows them to save face without admitting they are in the wrong.
Don't go in to your meeting expecting the worse with all guns blazing give them a chance to save face and back down.
I hope things work out in your favour.