I've only seen each edition (UK vs US) once as of this writing. The US version definitely misses out the scene where Choi (the killer) has sex with his cannibal friend's lover. However, this scene is part of the 'deleted scenes' extra, and is actually more drawn out than the scene in the UK version. The UK version only seems to 'hint' at his friend's cannabalistic tendencies, but the US version contains extended dialogue that leaves out any doubt that he eats human flesh. In short, neither version is completely 'uncut'. I have no knowledge of the Korean edition, sorry.
The UK version is the Korean version. The UK/Korean version cuts *some* of the violence and gore, but retains the rape scene. This was done so that it could be played in korean theatres. The International/US version retains all the violence and gore, but has the rape scene cut out although you can see this rape scene in the deleted scenes. The director prefers the international version and felt the rape scene did not fit the rest of the movie. It may also be due to cultural differences as the US has no problems with extreme violence in films but rape is considered somewhat "taboo/obscene" while that is the complete opposite in europe and asia.