Buckley's is possibly the best stuff on the free market. The children's variant tastes better but will probably not contain menthol, as small children aren't allowed to have that.

Codeïne is something you aren't allowed to have right now, because it supresses the cough reflex, wich will result in the snot festering in your lungs. This could result in pneumonia(!). (I work at the cemist's, we have to know these things!)

Maybe your local chemist can help you find something homeopathic that will help. Get well soon!