Strepsils are halal. The active ingredient, 2,4-Dichlorobenzyl alcohol, is a synthetic chemical and permissible for use. It serves as a mild antiseptic to help kill bacteria associated with sore throats and throat infections. The other ingredients listed, including sucrose, glucose syrup, and natural oils, are also halal.
And Allah knows best.

Sources:

• Askimam
• Islamic Portal
• Strepsils Middle East