Not a Goof
Continuity: "During the opening of the film, the ad on the top of Mark Thackeray's Bus changes from BOAC to Pan Am and back to BOAC"
After watching the opening title sequence, Thackeray jumps on a Bus, a number 15 (with a BOAC ad on the side).
Then 2 buses are shown pulling in to Bus Stops, the 2nd one (which has the Pan Am ad on the side) is a number 18. Thackeray isn't shown getting on or off this Bus. The first Bus can't be identified, but it pulls out quite quickly.
The number 15 Bus is shown again pulling in to collect passengers, and we see that Thackeray is already on this Bus, as a Woman sits down beside him.
So I conclude that why would he have got off a number 15 Bus, only to get on it again. That's very unlikely, as the Story is obviously showing Thackeray travelling to his first day in his new job.