I just rely on the travel insurance that is included with my banking account.
Are you T1 or T2? Are you travelling with insulin or other medication?
This organisation acts as a broker for travel insurance s
o take a look at this page.
Have you looked at any of the main comparison sites yet?
https://www.comparethemarket.com/travel-insurance/content/diabetic/
I'm sure that some other folks will be along later with some ideas and recommendations.