I was faced with the same question 2 yrs ago. I emailed the enquiry office but their reply was not helpful. I then spoke to the Jury Officer at the court, who said it would be no problem as the judges rarely went on too long without a break for fear of jurors losing concentration. He suggested the following:
1) Make myself known to the Jury Officer on the first day.
2) When called to court, tell the usher I am diabetic, he would inform the judge.
3) If I needed to test or eat I should write down "need medical break", raise my hand to attract the Usher's attention and show him the note. The judge, having already been warned, would know I needed the break ASAP. He would then adjourn for a short break until I'd was able to concentrate again.
Would only be totally excused jury service if your condition was very unstable or had no hypo awareness, e.g liable to pass out without warning.
In the event I had no problems. All judges adhered to lunchbreak times closely and the cases I was on never went on more than about 1.5 hours without a break. We had to take all belongings into court, so I could've tested under the desk if necessary.
When not on a case, never order hot food outside the official lunch break time as that's just asking to be called away from hot food you've aready paid for, whether diabetic or not!