Victoria Station sees its heaviest traffic on weekday mornings and early evenings, when the main approach roads 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.
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.
A quick note on group size and luggage at booking saves any surprise on the day: a saloon is comfortable for four with two large cases, and an estate, MPV or 8-seater takes the extra bags, buggies or kit that a bigger trip brings. Since school holidays and summer tend to be busier for journeys like this, booking a little ahead makes securing the right vehicle straightforward.