Is there a bike light law?

I assume no-one sees me and I use lights. Surely the best of both worlds?

Perfect !

I think the only (lesser) point being made is that use of lights can - with some - lead them to assume (if only partially) that they can be seen/are safe/r.

I subscribe to Tree Trunks' notion that it is best practice to assume every one is an idiot and cannot see you.