US DVD playback problems in UK
The fact that you see any picture means that your player is at least capable to detect and play 60 Hz DVDs.

When you say "horizontal lines" do you mean combing like this:



This is combing caused by incorrect deinterlacing. You player seems to do shoddy job of playing 60 Hz content on a 50 Hz display. Search for firmware update. Call/email the manufacturer. Also check deinterlacing options on the player and the TV set. Maybe your TV has a better deinterlacer, but is set to accept progressive signal, try switching it to "deinterlace" or "auto".

Sadly, I see the same combing when playing the Simpsons off a 60 Hz DVD on a 60 Hz player fed to a 60 Hz TV. One would think that after twenty five years of producing and testing DVD/BD players this should be fixed on all modern players (sigh).