Ian Tindale:
Why was the original film called "Passport to Pimlico"?

Terry Harper:
The point was that you needed a passport to visit Pimlico...

Yeah, but that doesn't really answer Ian's point:

I mean, a passport isn't to a specific place...

Note that people also say things like "your passport to adventure".
I think the answer is that back in the days when passports were
relatively rare and it was a big deal to have one, a passport *might*
be good only for a specific place. Presumably whoever chose the title
just liked the way it sounded by using "to".
--
Mark Brader "Look, sir, we can't just do nothing."
Toronto "Why not? It's usually best."
-- Lawrence of Arabia

My text in this article is in the public domain.