Luton to London sees its heaviest traffic on weekday mornings and early evenings, when the M1, A1(M) 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.
The pickup point is settled in advance so there is no confusion on the day, and any airport leg is flight-tracked, meaning the car follows your real landing time rather than the original schedule. Either side of the drive, expect the usual airport process — check-in and security outbound, or passport control and baggage reclaim inbound — both allowed for in the included waiting time.
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.