The police were his alibi. Not that he particularly wished to delay entry, but the longer the siege lasted the less suspicion would be thrown on the perpetrator, who also was priming them at every opportunity in what to expect within.
But this so-called alibi rests on convincing the police that Sheila is still alive and moving around the farmhouse. This in turn depends on Jeremy being present while the police are there, so the question still stands: how and why would his delay in mobilising the police and arriving at the farmhouse improve his situation?
Surely, if this was his plan, his interest is in making sure he is at the farmhouse before the police arrive, so that he can be certain they don't just barge into the main kitchen? How could he be sure the police wouldn't just do that?
People also say that he drove slowly to the farmhouse because he needed time to think, but I thought he'd already planned the whole thing? And anyway, it depends on what he needed time to think about.
And if he planned all this, why didn't he ring 999? If asked why his father hadn't done so, he could always explain that his father would want to keep the authorities away from the incident.