I dont see why you cant change a UK one, as I dont have one cant really say how though. Any good car audio installer will should be able to do it and charge you by the hour for labour.
As for mine, the Japs come with TV but its NTSC. So the TV tuner was removed, replaced with a standard PAL TV tuner and then the monitor had to be converted to PAL to work.
Once the TV tuner was in I had 2 pairs of AUX AV inputs extra on the TV tuner. One I used to connect up the DVD/MP3 changer and the other for a line in for my Ipod. Plus there was an extra AV input on the DVD changer, so I have another spare input for a playstation, camera etc..
The hard bit was converting the monitor to PAL.. you wouldnt have to do that, so in theory it should be easier for you.