"Muggles do know more than we give them credit for, don't they? I mean, you'd never know it was this roomy from the outside, would you?"
-- Molly Weasley, oblivious (CS5)
Molly doesn’t notice that the car has been magically modified, even when eight people plus all their luggage fit comfortably. A few miles down the road, Ginny realizes she forgot her diary — the diary which is really a horcrux — and they go back for it.
This is one of those moments in the books when the story would have changed dramatically if one little thing hadn't happened -- if her parents hadn't driven back to the Burrow to get the forgotten diary. Without that diary as the catalyst, there would have been no opening of the Chamber, no basilisk, no meeting with Aragog in the depths of the forest, nothing. Just a typical, ordinary year at Hogwarts. And with no diary being destroyed as evidence, there would be no clue to give Dumbledore the idea that Voldemort created Horcruxes. It's a good thing they went back for that diary, even though they were very late.
And speaking of being late, the drive from Devon, where the Burrow is located, to King's Cross takes at least four hours, and a lot more with heavy traffic. Somehow, Arthur made it in a beat up old Ford Anglia in far, far less time. After all, they got up at dawn, which in Devon on September 1 is about 6:30 am. In order to make it precisely at 11, with no traffic problems, they would have had to leave the Burrow a half an hour after that. The book, however, states:
It took a long while to get started next morning. They were up at dawn, but somehow they still seemed to have a great deal to do. Mrs. Weasley dashed about in a bad mood looking for spare socks and quills; people kept colliding on the stairs, half-dressed with bits of toast in their hands; and Mr. Weasley nearly broke his neck, tripping over a stray chicken as he crossed the yard carrying Ginny's trunk to the car. (CS5)
There's no way they made it out the door by 7, plus there's no way they wouldn't have hit rush hour traffic on the way into London. Molly might have forbidden them to fly, but she clearly didn't forbid a little wizardry to shorten the trip time.