One of the two wires in the lamp cord is neutral, the other is line. You can get an inexpensive tester that will indicate which of those two is hot. Is the plug on the lamp cord polarized, i.e. one blade of the two in the plug is wider than the other? This link provides some good info on that subject.
Hmmm, maybe you aren’t in the US, in which case this info is of no use to you. However, to complete a circuit, there must be a source of current (hot) and a return path for current (usually neutral.) I don’t know what that standard is for wiring in the UK and EU.