If you want prog scan you need a "high-definition av pack" for an NTSC xbox (it will work fine on a UK xbox). You can get one from
www.mx2.org (amongst others).
When you plug this into your xbox you can goto the video options menu on the main xbox dashboard, where it asks you about PAL60 it has a hidden option to say if your tv supports 480p. If you say yes then any uk game that supports prog scan NTSC (like halo for example) will output 480p.
You can get a program called dvdx2 which can play R1 dvds in NTSC progressive, and also R2 dvds in NTSC progressive. To do this you also need a copy of the "video mode select" program, this allows you to change between NTSC mode and PAL. I leave mine in NTSC mode all the time.
One last thing, when your xbox is set to ntsc mode the options added by using a hi-def av pack become more obvious and allow you to chose if your tv supports 720p and 1080i.
HTH
Owain