North Circular during rush hour??

Admittedly you'll be going against most of the traffic. You will want to cut up the A404 to Wycombe and get on the M40 - you should be able to get there by 4.30 before that snarls up. Alternatively, cut up the M25 instead (shorter...but it's the M25. ) Either way, you want to avoid approaching the Hanger Lane Gyratory from the North Circular itself, and in my experience the NC can be bad south of the HLG. North of the HLG shouldn't be too bad as you don't actually have to travel far on it, and the next major jam point is a bit further north. (Admittedly, this is based on experience about 8 years ago, I suppose things could have changed.)