https://screenrant.com/titanic-why-was-jack-dawson-in-england/
Titanic told the story of Jack and Rose but left a couple of questions about Jack's backstory, such as why he was in England.
He said because he was a drifter and wanted to see the world. He found himself in place to place.
One night he was sleeping under a bridge and another he was on the finest ship in the world breaking bread with some fine people.
Why not? He wanted to see as much of the world as he could see for pennies! And in an English-speaking country, he could pick up enough odd jobs to sleep indoors and eat food, so why not stay a while!
So can you imagine what a shitshow would have ensued if he'd lived, if Rose had run off with him, and she'd gotten pregnant? Imagine the fight they'd have, when he tried to convince a pregnant rich girl that sleeping in a park was no big deal...
He was heading back to America - maybe the cheapest rate from Europe to America was through Southampton. More importantly, does it matter?
Didn't he win his ticket in a game of poker?
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Yes, but he was in the seaport of Southampton with what seems to be the intention (along with his Italian friend Fabrizio) to get to America. Whether they had enough to get across in a tramp steamer or whether they would have stayed in Southampton to earn enough for passage the story doesn't say. But what good would it be to win tickets to America if they weren't planning on going there? It was too late to sell them.
What I've read is that the third class ticket on Titanic that Jack won in the poker game would have been equivalent to a first class ticket on a lot of second rate steamers, so - except for the Titanic sinking and killing him (bummer) - Jack thought winning that poker game was the score of a lifetime.