Okay, okay, modern life isn't exactly a character. But White Teeth doesn't have a clear choice for an antagonist. When we think about it (and we did), most of the conflict in this novel, even when it happens between characters, arises due to some aspect of modern life.
The Iqbal and Jones families live in diverse London at the end of the 20th century. This presents them with endless antagonistic moments, as first- and second-generation immigrants trying to find identities they can hold on to. They are also looking for ways to deal with the past in the present. The past just won't leave them alone, whether they want it to or not.