sm_jamieson formulated the question :
Just a thought. Its nice to have light in the CU cupboard !
What is the simplest acceptable way to wire (surface-wired) a light so it can
be on when the CU main switch is off ?
Perhaps without fitting a whole second mini-CU, henley blocks, etc.
e.g. via a fused switch directly from the main incomers.

There is no other way I can think of. Unless you think in terms of an
emergency light, or some sort of battery powered light.