The archive footage was the very opening scene where it shows him playing in a match, which she is dreaming/watching on TV. I don't think it's referring to the poster as a credit wouldn't be needed for that. That's also him in the poster, not a fake.
Body doubles/lookalikes were used at the end. This is shown in the making of, which I recall seeing several years ago, it showed the faces of the doubles. I can't remember the reason why they used doubles, if they even went into it. No idea of scheduling conflicts or if the Beckhams just didn't agree to be part of it (not knowing what a success it would be). I doubt there would be contract issues.

I think he Beckhams were in two minds whether to appear in the film.
Its that man again!!

Director Commentary says they weren't in the movie. I think there was a scheduling conflict or something with the Real Beckhams so they just used doubles in the airport scene at the end.

Yes. I use that one a lot.
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Good emoticon; me, too. Only an insider would know for sure, because we folks don't live on that level and would only guess.

I don't think the archive reference would include posters. It would be any clips of him on screen . . . such as playing football/soccer, etc. I believe they show them watching him on the 'telly?' That would be where the archive footage would come from.
And then the actors playing them in the airport scene.
Them not being personally in the movie could be due to scheduling. Or how much it might have cost to 'hire' them for the day.
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