Prove me wrong and come up with a reference from a national aviation authority, not an in-house airline manual, that requires the dimming of lights for take-off and landing. And I can assure you, there are many airlines that don't bother 'dimming'.
Taken from Chapter 32 of CAP 768:
"
15, CABIN LIGHTING
The dimming of interior cabin lights, particularly when taking-off and landing at night is recommended."
As this is a recommendation, dimming of interior cabin lights at night is I guess, at the operators discretion only.