I'm type 1 and have had good quotes over the last few years from AllClear Travel who seem to specialise in cover for folks with pre-existing conditions. Found out about them from the Diabetes UK website. Their address is: https://www.allcleartravel.co.uk/
Open an account with nationwide, pay in £750 a month and get free insurance. You must be on insulin only and not take any other glucose control medicine such as tablets. It's saves a fortune and is annual European cover.