Weekday rush hours are the main thing to plan around for Heathrow to Milton Keynes, since the M4, A4 and M25 get busy with commuters as well as airport traffic. An early-morning or early-evening weekday trip is worth a little extra buffer; things generally ease off in the evenings, at weekends and during school holidays. The fare stays fixed regardless of which of these you land in.
Nothing about the pickup is left to chance: the meeting point is agreed in advance, and on airport legs we track the flight so the timing matches your real arrival, not the timetable. Plan for the usual airport process around the journey itself — security and check-in outbound, passport control and baggage inbound — which is exactly what the free waiting allowance in your fare is there for.
Sort out group size and luggage when you book, not on the day: four passengers and two large cases fit a saloon well, while an estate, MPV or 8-seater handles more — extra suitcases, pushchairs or sports bags included. This kind of journey sees more demand over school holidays and summer, so booking with some notice makes it easier to get the vehicle sized correctly.