Here in the UK we use cable like 2.5mm Twin&Earth (2.5T&E) or single cores 1.5mm.
Where the size stated is the diamiter of the copper core
Not the diameter -- 1.0, 1.5, 2.5 etc. are the cross-sectional area
of the conductor, in square millimeters.
The pre-metric cables (before 1970) use a size designation based on the strand diameter, e.g. 3/.036 is a conductor consisting of three strands, each of .036" diameter.
Here is a comparison of American 12/2 w/gnd Romex and British 2.5 sq. mm "T&E" cable:
Note that the ground conductor on the U.K. cable is a size smaller than the insulated conductors, 1.5 sq. mm on this 2.5 cable.
You'll note also that British T&E doesn't have any paper inside the outer sheath like Romex.
[This message has been edited by pauluk (edited 07-23-2006).]