Heathrow to Victoria 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.
We agree the exact pickup point with you before the day, so there is nothing to work out when you travel, and any airport leg has the flight tracked so timing follows your real schedule rather than the printed one. Build in the normal airport steps around the drive — check-in and security outbound, passport control and baggage on arrival — both covered by the waiting time already included in your fare.
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.