A little more information ... possession of laudanum (or any other opium derivative) without a prescription was outlawed by the Dangerous Drugs Act of 1920.
So, I now think it unlikely that the lady in question had that substance available in 1928 (and, if it had been prescribed for her, would the family have used the phrase "drank poison" to describe her action?). If it wasn't a medicinal substance, then that leaves only the nasties - rat poison etc. Not at all nice.
Her youngest child was born Q3 1927, and she died Q4 1928 - does anyone think post-natal depression could have been a factor, or is the gap too big?
Mike