In an interview published on 11 September, 2020 for Daily Mail UK, Carol Drinkwater, the first Helen, reveals why she left the show. She felt the role "needed more meat": "I suggested she might visit outlying farms, assist the elderly, lonely, join a charity organisation...."
She is also quoted as saying, "I felt that there was little more I could give to the role. The BBC wanted to keep Helen in her place and I felt she needed to be more feisty. I needed them to give more meat to her scenes."
The producers rejected her suggestion on the grounds that they didn't want to depart from the books. So she left- and moved on to a successful career as a writer.