Where can you buy tickets for a film premiere in London?

Hi I am also looking about how to buy tickets. I have searched every where and havn't been that successful. I was told that you can't buy them you have to receive them. But i am not sure whether that is true or not. I think that you may have to search for competitions that give them away to winners. And if the worst comes to the worst you could always just go and see the stars in Leicester square on the red carpet! If i find anything more i will let you know. xxx