No, it isn't.
The clock will stop at 7:50pm no matter what you do and will not restart until you leave London. I guess this was intended in part to make sure that you actually go on a journey after all (all kinds of complications would arise code-wise regarding to the end of the game, should you be able to stay in London for 10-50 days or something) and in part to allow the new players to accustom to the UI without missing their very first trip.
The option to buy your tickets before boarding the Amphitrite Express is only available before you complete your first journey around the world. After you arrived back in London at least once, you will be shown a shortened intro in all your subsequent games. The good news is that until you don't finish your first game, you do have the option always - whether you pack super fast or not.
Finally, there are an awful lot of decisions that affect your character, ESPECIALLY in the intro and in the first third of the game (Europe). It's not at all guaranteed that you always end up dependable by rushing to the ticket booth before boarding - you can do better or worse too.
Have fun!