There is no absolute answer to your question - it all depends on how your employer sees the issue/ what is in your Contract of Employment and/or any qualifying documentation.
From the point of view of Statutory Sick Pay, a day on which you turn up for work and then leave early (no matter how early) because of sickness is not counted as a sick day. Most employers, as far as I know, operate the same sort of system. It would be possible however, to record 'half-days sick' or even 'hours sick' if .they wanted to go that far.
Some staff are allowed time off for dental treatment or a doctor's appointment, others have to take this out of their annual leave or sickness entitlement - it all depends on the firm.