What do I do if I want 18 rated games? (mature in America)?

I live in the uk and I want 18 rated games. I am allowed 15 rated games but not 18. I have grand theft auto iv, but I can only get mine from Australia because there they are rated 15. (MA15+) I have tried loads and they talked about how supernanny got two kids and 1 had saw these games while the other hadn't, and the blood pressure stayed the same on the children who play these games, but dramatically increased as she showed rem both a picture of the war. Please help. I love them so much and I really don't want to fall out with them. I hope they aren't reading this now. They mean the world to me.