I'll let others tackle the code issues, and just describe a bit of theory as it relates to US wiring.
As you know, in both England and the US, power is distributed at higer voltages, and then stepped down to lower usage voltages using a transformer. Except for rare instances in industrial wiring, the secondary of the transformer is always somehow bonded to ground so as to define all other voltages in the system relative to ground and to provide a (hopefully) defined fault current path in the event of a ground fault.
Once you tie a point on the secondary of the transformer to ground, all other points have a defined potential difference to ground.
In the US, the most common supply for residential wiring uses a 240V secondary on the transformer, and bonds a center tap on that secondary to ground. Three wires from this secondary are brought into the house; the two terminals of the transformer (the 'hot' wires) and the center tap terminal (the 'neutral' wire). The voltage between the two hot wires is 240V (nominal); the voltage between a hot and a neutral is 120V (nominal).
Separate from these 3 supply wires, you have your safety ground or Equipment Grounding Conductor. This fourth wire is not supposed to carry current, with a couple of exceptions. The EGC is supposed to carry fault current, is permitted to carry small transient currents from surge suppressors, and is permitted to carry small currents from various grandfathered loads (meaning under current code the EGC cannot carry current, but in prior versions of the code it was permitted.)
Because the EGC is not supposed to carry current, but is supposed to be at the same potential as the neutral conductor, these two conductors are bonded together at one and only one location; the service entrance box.
When you looked at the wires coming into the house, it is quite likely that you saw two insulated conductors and a third wire which appeared to simply be a carrier or messenger wire...quite likely this carrier was also serving as the neutral conductor. So you had three conductors coming in to the service entrance.
You will also have a grounding electrode system connected to the service panel. So that makes your four wires. Any subpanel from the service panel needs to be fed with these four separate wires
-Jon
