Originally Posted by
rcspeirs
You're planning to do Oxford and Warwick Castle the same day? Too ambitious for my taste - most days the castle is closed by 1700, so you'd presumably want to get there early afternoon at the latest. Leaves precious little time to explore Oxford, and that's even without the time faff of collecting the car and driving to Oxford.
A brilliantly British understated way of saying that it will be impossible to see Oxford and Warwick castle on a day when you are starting in London, unless you want to start very early indeed, when you won't be able to rent a car.
Onto the question in hand. Renting a car in London is not difficult. Depending upon where you rent it, you may or may not need to worry about the Congestion Charge. For example there's a large Hertz near Victoria and you wouldn't go anywhere near the Congestion Zone on your way out of London. So it depends where you are staying. And, even if you have to pay it, it's not much split between multiple people.
Personally, I think if you want to do this, I would pick the car up the night before, then leave at 6am, arriving in Oxford before the rush really starts. Accept Oxford's horrific parking problems and aim to leave by about 1pm. That should put you in Warwick in time to see the castle, with a little leeway for a) traffic, b) getting lost and c) finding Oxford more interesting than you expect.