That seems to be the problem here - most of the candidates are foreigners.

How about Garbo, whose exploits shortened WW2, he was a Spaniard.

If forced to choose someone without whom we would not be here, I would not choose Churchill, but Stalin.

Nelson and Queen Victoria. Nelson defeated the other naval nations leading to Britain 'ruling the waves'. Victoria reigned over the largest and greatest empire ever seen. The innovation of the Victorians and the power of Victorian Britain have no real current rivals. Britain was the world's powerhouse and current superpowers do not really rival it.
I know there are problems with this. I'm not interested in Nelson thinking that slavery was okay - most people did. He is still England's greatest hero.
Victoria had quieter spells in her reign and married a German - don't they all? A lot of people were poor in Victorian Britain but Victoria was charitable and virtuous when it was entirely necessary. She also allowed parliament and business to work without interfering too much. The Victorian age is named after her for a reason - maybe symbolic - but what she symbolises is so great that Britain deserved the Great. So far removed from today. Those Victorians got things done.

With Henry's reputation If she had been a hooker she would have had more chance.

But for Stalin, there are 20m+ people who would be here (or would have died of natural causes).

You really haven't thought this through.

Newton or Hawkings?

Boudica or Pankhurst?

There is no physical evidence that she existed and she lost anyway... According to the Romans.

You might as well nominate Harald Godwinson. He definitely did exist, became king and then quickly had to thwart one invasion and then tackle the Normans days later.

He was a bit unlucky really.