Measuring the voltage at the lampholders (first and last on circuit) is the first course of action, but you also need to measure the voltage between the Neutral and the Earth conductors to.
Write these values down, then measure, as BR says, the value across phase and neutral on the input side of the main isolator, write this down, then measure the output side of the MCB for the circuit concerned.
The two figures you just got from the consumer unit should match, the volt drop here should be only tenths of a volt if that.
Between these values and the furthest point in the lighting circuit, the voltage difference should not exceed 9.2V.
Write all these figures down, ensure you label the figures correctly, then post them here for us to have a look at.