I found the Diabetes UK quote to be quite expensive, but a company called Karma was good. They have a list of conditions (including diabetes) that if you have it and it's well controlled and you've not been in hospital because of it then there's no increase in premium. Might be worth a try.
The issue with travel insurance can be excesses and how they are paid/claimed and worse still claims and the settlement.
Many of the cheaper policies are cheap until you need to claim, at which point the excess makes the policy more expensive. Many insurers expect you to pay in full locally, then claim back once you're in the UK.
I really don't know how this works if the claim is a big one, if this is settled directly with the hospital/doctor, but it could blow a temporary hole in any holiday spends you had planned.
Of course we hope never to use the insurance, I feel though so few of us ever look past the premium and of course in our case will they cover diabetes. If the number looks ok and the answer is yes, we go with them, without questions on excess, what exchange rate is used, who pays and when.
At the time you need to claim on your policy its not a good time to discover you have ?20,000 of medical claims, ?300 left on your credit card, ?500 in cash and your medical cover won't settle with the hospital or they won't accept your insurer settling with them.
Why does my mind always look for the gotcha in everything?
The old line of if a deal looks too good to be true, it more often than not probably is - if its really a superb deal, PM me, but don't tell anyone else
