I've seen insurance for Type 1's change dramatically through the years. When I got my first car in 1978 soon after deing diagnosed, I was charged £600 per quarter for 3rd party only (this was 1978 remember, when petrol was under 50p a gallon). I had to have medicals and letters from my doctor for each insurer so even attempting to shop around was a major excercise. In those days Diabetes was a relatively unquantified risk so they whacked up the premium to make absolutely sure they wouldn't be out of pocket.
These days, the risk assessment is made by the DVLA medical section and reflected in how many years your licence is issued for between renewals. Insurers don't even ask what's wrong with you any more. Most just take a 3 year licence as an indication of low risk, although I always make sure to tell them what's wrong with me, so they can never pull the old 'you never declared that' one on me.
You say that telling your insurer when you were first diagnosed made no difference, but like us shopping around for a new insurer, the insurers themselves will shop around for a new underwriter. It could be that your insurer has picked a new underwiter, whose favourite tool for generating revenue is medical conditions. Also could it be that as you were only recently diagnosed, you don't yet have enough clean history to get a 3 year licence? That would give an insurer who doesn't like medical conditions just the excuse he needs for making some easy money out of you.
I don't think an insurer would be commiting an offence to charge more for insuring an increased risk
These days there's so many little scams they have to put your premium up, they don't have to focus on a medical condition . When I tried to renew our two cars with the very same insurer this year, I just happend to notice they had us down as only havng ONE car in the household on one of the policies. On speaking to the agent I asked if he could get that corrected to two for the new documents, as it must be their error as they know we have two cars insured with them which were both insured at the same time on one phone call. Incredibly the agent said that will be an EXTRA £30 (8%). Apparently for this insurer (not others) having 2 cars is a somehow a significantly bigger risk. Yeah right!
It's really worth shopping around very carefully as every parameter is considered a different risk by different insurers, and these can vary each year.