Jeremy attended a prestigious public school, had O-levels and attempted a maths A-level. He's not a genius, but I would say he is of above-average intelligence. As I have commented before, if he were going through the system today, he'd probably end up at a university somewhere or maybe take a degree at Cirencester.
He passed the selection exam for the upper school at Gresham's, where he showed some ability in maths and geography. Later, he was admitted into the Sixth Form at Gresham's to take A-levels. He performed indifferently and switched to a local FE college, where he continued with a maths A-level.
I merely make the observation that he was and is manifestly of above-average intelligence. He's not a stupid person, let us put it that way. I believe he was intellectually capable of executing a murder plan of this kind. Whether he actually did so is another matter.