Europcar at Victoria is open both days at the weekend. Hertz is open Saturday at Victoria, Avis is open Sat and Sunday morning (you don't need it to be open to return the car if I recall correctly, there are drop boxes for the keys).

If 4 people get the tube out to LHR and back that is going to add to your costs a fair amount - I would just hire at Victoria (or similar location). Driving out Friday night is likely to be worse traffic than Sat or Sun morning IME, but who knows - as others have said, traffic is inevitable. Just find a good music station and zone out (ideally, as the driver, keep a bit zoned in tho!)