I appreciate your problem: it may be because the law only requires emergency lighting to be 'sufficient' based on risk assessment, which is a reasonably flexible piece of string.

You could ask your local Fire Service for advice, they might be prepared to help. Since they're the ones who would act in the event of a breach you can be reasonably confident that whatever they say goes,

John

Your basis for judgement can be based on BS 50172@2004 Emergency escape lighting systems

I haven't checked if this is the latest version of the standard, in anycase I doubt the principles have changed

Escape lighting should capture the following

1 Illumination for fire route escape signs (appreciate, signs can be self illuminated or fluorescent)

2. Provide light onto and along escape routes

3 Provide light for visibility of break glass call point and to see fire fighting equipment

If memory serves me, the minimum lighting level 1 lux