DCUK has some advice about insurance here, although I am not sure it covers your situation:
http://www.diabetes.co.uk/finance.html.
When I was living in the Middle East for a few years, I bought my own BUPA insurance that covered the cost of local treatment. That was a couple of decades back, but you might want to check them out in case they still do this and will cater for pre-existing conditions.
(For some of that time in the Middle East, I did not need to purchase my own insurance because it was provided by my American employer. I assume you do not have such provision from your employer.)