I have noticed that some cold remedies are now sold in "sugar free" versions. Otherwise just read the info panel and avoid the sugar. I bought some sugar free Halls in Tesco's the other day for my sore throat. I've had Strepsils in the past.
Since nothing will actually cure a cold, it doesn't much matter what you choose to control your symptoms. Good old paracetamol works as well as anything and any antihistamine will help decongest. We are not supposed to use pseudoephdrine decongestants, but since I know my BG is NORMAL and my blood pressure is also Ok, I do take decongestants if I feel the need. It's my own choice and my own risk.
Hana