Heathrow to Exeter sees its heaviest traffic on weekday mornings and early evenings, when the M4, A4 and M25 are shared between commuters and airport travellers. If your journey falls in one of those windows, building in some extra time is sensible; outside them, and at weekends, the roads are typically quieter. Either way, the route is planned around the traffic on the day, not around a fixed schedule that ignores it.
Your pickup point is confirmed ahead of time, removing any guesswork on the day, and flight tracking on airport journeys means the car adjusts to when you actually land rather than when the schedule says you should. The usual airport steps still apply either side of your drive — check-in and security, or passport control and baggage reclaim — and the included waiting time is there to cover them.
Groups and luggage are worth mentioning at booking rather than working out on the day: a saloon suits up to four people and two large cases comfortably, while an estate, MPV or 8-seater gives more room for extra bags, buggies or sports kit. School holidays and the summer season tend to bring more group and family travel on this kind of journey, so booking a little further ahead helps guarantee the right-sized vehicle at a time that suits you.