Weekday rush hours are the main thing to plan around for Heathrow to Birmingham, 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.
On the day, your driver confirms the pickup point in advance so there is no last-minute searching, and for airport legs the flight is tracked so the car is ready whether you land early or late. Allow the usual airport routine either side of the drive itself: security and check-in on the way out, and passport control or baggage reclaim on the way in, both of which our free waiting allowance already covers.
It helps to flag your group size and luggage when you book rather than on the day — a saloon comfortably takes four passengers and two large cases, and an estate, MPV or 8-seater covers anything bigger, from extra suitcases to pushchairs. Demand for this kind of trip tends to rise in school holidays and over summer, so an earlier booking makes it easier to lock in the vehicle you need.