If you ask in Boots, they have a special ‘diabetic’ cough mixture which is absolutely foul! It tastes like liquid tarmac.
I like Sugar Free Mentholyptus, Sugar Free Strepsils, and there’s also another throat lozenge that’s sugar free whose name I’ve forgotten.
Another option is the children’s section where they often have sugar-free meds. Personally if I have a really bad cough as I did last year, I just buy the best/most suitable one and only take it at night, ignoring the sugar. Lots of them are pretty rubbish so if I find one that works, I take it.