Ann admitted that she hadn't seen her parents since she left London.
Is the above sentence OK or do I have to write
Ann admitted that she hadn't seen her parents since she had left London.
The sentence is fine. The visit happened at a particular moment in time as well as Ann's admission. The activity or inactivity that pertains to the span in-between these two occurrences would in the active mood be written in present perfect. As you are supposed to shift the time backward in reported speech, though not in all cases, you use the past perfect.
Ann admitted that since she left London (reference point in time) she had not visited her parents.