I also calculated, most central place in UK, if you want a light to be visible from the whole place, it needs 33km of altitude (not super sure about my calculation but it sounds about right). If you just want it to be seen from half the distance, it only needs 4 times less (square root) so from about 8km high you can be seen by the whole "dense population" part of england, for example, if sky conditions are perfectly clear.
I have seen the whole iberic peninsula once from the air, at night, perfectly clear sky (a front had passed and washed the whole country), and lights everywhere to make it easier. I could spot Barcelona before I reached Madrid.