Well, strictly speaking, it’s neither the Parliament, nor Big Ben. Big Ben, for example, is not the tower. Or even the clock. It’s the Great Bell, which is in the clock, which is in the Elizabeth tower, next to Palace of Westminster.
And the Houses of Parliament (both the Commons and the House of Lords) meet in that palace.
But that’s just being pedantic, and picking nits. Which is most unlike the denizens of those parliaments 🙂