The severity of voltage drop depends on the resistance between the substation transformer and the point in your wiring where lighting and power splits - normally fixed lights are on a separate circuit so this point is at the consumer unit. A plug-in light experinces the full drop of the final circuit cables too, so is more noticably affected, particulary if the distance ot the consumer unit is long.
If you are a long way from the sunbstation, the companies supply impedance may be a good fraction of an ohm (maybe more than half an ohm in some cases... 30A supply, long rural feed etc.) so a 13A kettle might drop 7V, a dimming effect enough to be seen. Of course if your old house was next door to the subst, then the resitance could as well have been <0.1 ohm, so dimming not noticable. In any case, not sinister unless it suddent goes from not noticable, to really bad, in which case something is going high reistance, which can be an early warning of a failure.