I had an interview at Southampton, but most people don't. And yes interviews can show things, but people can be taught to do well in interviews making them useless in some cases anyway. And you think people won't lie in person ohhh they do. I've watched them come out of the interview room and tell me some of the things they've said.
I'm glad i had an interview, i always do much better when i'm able to talk to people, but i can see why they don't bother. I wish they did, but all the people on my course (6yr @ Southampton) were interviewed and it didn't make that much difference....
And UKCAT is required for BM5/6...not sure about BM4 (graduate entry)