Not being promoted in favor of Meghan, meant not getting a substantial raise and being able to help his parents. He was certain to get it if Meghan wasn’t around.
Obviously observant and organized, he knew Meghan’s routine; even how she managed her kombucha drinking, which he had convenient access to before she arrived to work every morning.

Backstory…

Forrest grew up with his older brother, Nathaniel, on Northumberland Coast in a fishing family. His father, Gordon, was a 4th generation commercial fisherman and his mother, Charlotte, was his high school sweetheart and a stay-at-home mother.
It was always assumed Forrest would follow in the family business as his older brother had, especially after Nathaniel died from a freak case of Cholera.
Although extremely appreciative of the life provided by the family business, not being such a formidable figure as his father or brother, Forrest never saw himself on the high seas.
As a result, he concentrated and somewhat excelled in academics. And although it crushed his parents, having already lost one son to the perils of the business, they supported Forrest in his white-collar pursuits.
The unselfish concession of his parents’ familial dream drove Forrest to graduate from Lancaster University with a degree in Biochemistry with honors and pursue a high paying position with a major pharmaceutical company to help sustain his family.
Recently his father, Gordon, had be diagnosed with malignant tumor in his brain and was no longer able to work. Unfortunately, this scenario was more common in the small fishing villages that dot the coastline. Maybe due to higher than ever mercury contents found in the species being fished.
*He had recently begun to lace Meghan’s kombucha at work with thallium to eventually kill her, and why it was detected in her system, but not enough to kill her, yet.
But he did not have access to it the morning of the murder and was no where near the crime when it happened.
And although he had access to deadly nightshade to and from his parents, that day he was check-in at Wexham General in Northumberland, which is a 4 hour drive each way.