what are the restrictions for diabetics.

Diabetics were not in the shielding group (unless something else qualified you - though some doctors have been adding some diabetics to shielding lists - but only some doctors and I think mostly for those with HbA1Cs), so unless you've specifically got the shielding letter yes you can go out - though if your husband is shielding I don't know what the rules for that are if you do go out.

Diabetes on its own, as long as you are reasonably well-controlled, doesn't require shielding according to the Govt, but we are in the highly vulnerable group so advised to be extra careful about cleaning and social distancing, etc., as the advice for a partner of anyone else who is shielding to reduce the risk to them.
However, the UK govt never got to grips with the issue of multiple risks, ie diabetes plus mild asthma plus mild heart disease which research shows increases the risks. So as I am retired and only a few months off 70 with all 3 conditions I decided to shield anyway.

Has your husband had a letter? I would be more worried about bringing the virus home to him. My husband is self shielding, because I am.

If you are doing online shopping, you should do it in your husbands name/email. You should ask your doctor if you can be put on the list. Yoou should be able to have medications delivered.