Balanitis. This is due to microorganisms fermenting a tiny amount of urine retained under the foreskin. Get in the habit of blotting or washing off that last drop of urine after you pee. Castellani solution is very effective. Circumcision cures it.
Balanitis? Smegma alone doesn't do this. Maybe balanitis, the medical term for inflammation of penis head and under foreskin. It's often due to overgrowth of bacteria or yeast; when there is no obvious cause, it's called "nonspecific" balanitis. Simple hygiene often is enough -- retract foreskin and wash 2-3 times daily. If that doesn't help in 2-3 days, see doctor, either your GP or NHS GUM clinic.