Got my HID kit 6000k installed last month, and the guy, who was doing it said that the best and the safest option is 6000k. You won't have any problems with MOT or the police.
Some people have 8000k, like VR6Joe, and it's not THAT bad...It's kind of borderline in terms of legality, so unless you want to "test" the police or the MOT guys, you're better off with 6000k.
More than 8000k is just dumb and you're looking for trouble. This guy said he installed 10000k kit for someone, and the driver didn't even get home. Got pulled off after a few miles, lol.