Not sure if this helps but .. In Australia a employer does not have the right to ask you what caused your sickness/days off.
My doctor would never put a reason for sickness down in a sickness certificate., only that I was suffering 'from a medical condition' (which covers a multitude of possibilities). If hypothetically I was suffering from a sexually transmitted disease, does anyone outside of my doctor have the right to know ?? There are privacy laws here. Of course privacy does not apply when the doctor considers the illness/injury etc is work-related.
Of course the doctor may not be able to offer a definitive diagnosis beyond say 'viral illness' and again what business is it of the the bosses to know that.? Can they legally demand you explain? Are they exceeding their authority (= bullying)?
And the problem with diabetes is that a viral illness is likely to send up the BSLS as a secondary effect. Yes you may need longer time off work to get the BSLs settled than if it was just the viral illness to recover from. But I would guess that that is where discrimination could be argued if need be. Also in some companies they allow a certain number of sick days per year without a medical certificate but we are only talking about say, 4 days per year.
I recall a discussion with some nurses back in the day (1980s) where the matron required that the nurses put down a diagnosis of illness on their sick leave forms (with no legal standing for the matron to make such a demand). Most of the largely female nurse population would put down 'orchitis' (inflammation of the testicles) as their illness. I hope the matron got the message !!
So legal advice about UK privacy laws may also important plus whether you received any orientation/paperwork when you started work as to your rights and responsibilities. Very occasionally, employees require that all illness/injuries occasioning sick leave are divulged. You would have had to specifically agree and sign such an agreement. But that would be unusual. at least here in Australia.
Sometimes it may be a day or more before being able to get to see a doctor for a sickness certificate. (either because of delay in obtaining an appointment or because it was physically impossible to get to the surgery without major embarrassment (nuff said).
I found some wonderful wording for such a situation, wording like this or similar:
"From information received from the patient, which I believe is true (or consistent with my subsequent findings), the patient was unwell for X days(dates) prior to this consultation, Today --/--/-- , he/she has a medical condition which renders him/her unfit for work from date Y to date Z inclusive". It is helpful of course if wording like this meets the approval of say, the GMC.
In my somewhat limited experience I have yet to find an employer who has quibbled about not including the X days before medical certificate in the sick leave payment.
Also a doctor might be able to provide a medical certificate for a time when you consulted him/her, but thought you could battle on at work but had to take time off. My routine in this case was to ring the surgery, explain the situation to the receptionist and see whether the doctor could supply a medical certificate based on the prior visit. or needed to see me again.
My Best Wishes to you and I hope commonsense and justice prevail, as they most definitely should.