I'd just like to interject for a moment. What you're referring to as Big Ben is in fact, Westminster Clock Tower, or as the government has recently taken to calling it, Elizabeth Tower. Big Ben is not a clock tower unto itself, but rather another venerable component of a fully functioning bell tower system made useful by the tower's clock, and acoustic properties.

Many Londoners hear Big Ben every day, without realizing it. Through a peculiar turn of events the tower which houses the bell is often called "Big Ben", and many of its listeners are not aware that it is the bell.

There really is a Big Ben, and these people are hearing it, but it is just a part of the system they keep time with. Big Ben is the bell: The big hollow dome of metal that makes a sound when struck. The bell is an essential part of a bell tower, but useless by itself; It can only function in the context of a complete bell tower. Big Ben is normally used in combination with the Westminster Clock Tower: The whole system is basically Westminster Clock Tower with Big Ben added, or Big Ben in Westminster Clock Tower. All the so-called Big Ben pictures are really pictures of the Westminster Clock Tower that houses Big Ben!