Big Bens absolute location is as follows:

51° 30′ 2.2″ N, 0° 7′ 28.6″ W

It can be found here:

The closest and easiest tube station to find Big Ben is Westminster station which is on the Circle, District and Jubilee underground lines. Also Embankment tube station is within walking distance of Big Ben (roughly 5 minutes).

The clock tower itself isn’t actually called big ben, the bell inside the clock which creates the chime is called big ben.

The big ben tower the worlds largest four faced clock which chimes and celebrates its 150th birthday during 2009.