Don't see why not but will be interested in answers you get because my husband is type 1 and I was just about to include him on our new lorry's insurance.It had never occured to me that it might be an issue.
Thing is, if you're type 1 diabetic, you don't have a normal license, you have a temporary one that gets reviewed depending on your controlling of your blood sugar levels..
It is now possible to get approval for C1 ( up to 7.5 tonnes) subject to various sensible criteria. and they are still campigning.
check blood sugar levels before setting out and call Diabetes UK hlepline for most up to date info and advice.