I have been with directline since before diagnosis. Informed them when I was diagnosed but have not seen any jump like you describe.
I have seen the usual slight increase come renewal time and on occasion I have simply cancelled the policy then renewed with them on the website all over again for more or less my original rate. I think this is a usual business thing and nothing to do with the T1 since the wife has seen similar too.
But as noblehead says just shop around a bit. While you are required to inform them they are not allowed to discriminate though proving it can be tricky I guess.
/A