One thing to remember is that most of the time, the warnings on packets telling you 'this is not suitable for diabetics' is just the manufacturer covering their arse. Generally all it means is it's not been tested on someone with diabetes so they can't guarantee it. Ask the chemist but generally it's not something I worry about.
I wouldn't bother with cough medicine - generally it doesn't work and it's just sugar and glycerol. Pholcodeine may work better but it's still syrup, at the end of it.
When I have a cold, I have a very firm treatment regime that works brilliantly for me:
*If it's a bad'un (shakes, fever, blocked sinuses, headaches etc.) then I'll live off the all-in-ones for the day. Own-brand ones work fine, you don't need expensive Lemsip. I also prefer the tablets to the drinks but that's a personal preference based on how quick I want things to work and also the fact that tablets don't need flavouring/sweetening.
*Vitamin C tablets - no proof they work, but I find chewing them helps with a sore throat
*Chloroseptic spray - for the really bad sore throats. Otherwise, the all-in-one or paracetamol do the job.
Finally, my special secret which has NEVER failed me.
Gamma-aminobutyric acid (GABA).
What's GABA, I hear you ask?
It's a bodybuilding and mood supplement. It's supposed to act a bit like valium and it also improves muscle tone. It is the precursor to human growth hormone. And interestingly, if you feed diabetic mice enough of it, they stop having diabetes. Your body usually makes GABA naturally but the GAD anti-bodies responsible for islet cell destruction in T1 also prevent you making it.
Unsurprisingly I haven't cured my own diabetes with it, but I've personally found if I take a lot of this stuff (1400mg every couple of hours), I can wake up with a cold and it'll be gone, completely by the evening. For me, it is INCREDIBLY quick at resolving colds - I used to be the sort of person who'd have a cold lasting a week, now with this stuff it tends to last no more than 24 hours.
Don't ask me how it works or why, there's no research into it. My theory is that the symptoms of a cold are caused by your immune system destroying infected cells - GABA, as the precursor to HGH, stimulates cell growth so perhaps taking GABA means I'm regrowing these destroyed cells more quickly, meaning I don't get the symptoms. For me, it definitely works and it's definitely nothing to do with the placebo effect. I used to take it every day and it was only after going through a whole winter without catching a cold (despite having a girlfriend who was more mucus than person for most it!) that I realised I hadn't had a cold or any illness in that time.