OK, so...I think I've figured this out.
The album version of Tomorrow is only available on Your Arsenal.
The US single version is a radically sharper mix, done by Steve Peck, that includes a longer, clearer piano outtro.
The video version is a third mix; still the same as the Steve Peck version but this one deletes the piano outtro.
The versions on both the 2001 Best Of and the 2011 Very Best Of are the same version: the Steve Peck mix but without the piano.
In closing, the actual US single version (with the longer piano ending) has never been collected on a compilation or released outside of the original limited edition single format...which makes it an out of print rarity.
This interests me because one of the key selling points of the 2011 Very Best Of was the inclusion of the U.S. version Steve Peck remix of Tomorrow...which it didn't actually have; what it had was the video version of that remix which had actually already been out commercially on a pre-existing compilation for ten years.