However, the writer possibly saw the being forced as being anterior to the actual leaving.
It's also possible (though admittedly unlikely) that Dr Noel was the owner of the practice, and continued that ownership after his departure. Alternatatively, he perhaps left London (as a place of residence) but continued to work there.
You will find that many native speakers are rather more cavalier about the usage of the past perfect than some grammar books suggest they should be. In real life, this does not matter; in most contexts, no confusion is caused or felt. In examinations in English, it is safer to stick to the rules.
Student or Learner