In theory, DVDs coded "1" will play on U.S. DVD players, while code 2 is for the UK. DVDs are also recorded in NTSC or PAL, the latter for the UK.
However, we have sold hundreds of code 1 DVDs that are NTSC recorded to customers in the UK, and have not heard a single complaint.
It's not clear to us whether DVD players, or perhaps the
DVD discs, are effective blockers of wrong codes. We also cannot seem to get a definite answer about whether PAL vs. NTSC makes a difference to DVD playability.
We did ascertain that older DVD players are less likely to play DVD "9s", which have dual layers. If a DVD has three hours or more of recording on it, it's probably a 9.
This probably hasn't been a great deal of help.