If that were the case, ordinary pocket torches and mobile phone camera lights would be forbidden.

Not that I know of, but I would have thought that self-contained portable lights wouldn't be in scope of BS 7671 anyway - so chances it wouldn't have anything to say about them either way.

There is a more specific standard for temporary events though - BS 7909- I'm not familiar with its contents in detail - so that might be worth a check if you have access to it. Certainly on-site generation is common, along with (large) battery systems that allow the generators only to run part time at full load for better efficiency, likewise maintained emergency lights I'm sure have been used - so I don't see any blanket ban on batteries or local sources of power being that likely.

The IET also publish a Guide to application of BS 7671 and BS 7909 - which might be worth a look (there's a new version in the pipeline, but I don't recall any mention of battery lighting in the draft for public comment that I saw).

It may just boil down to risk assessments - how practical is it to ensure that the lighting will work when it's required - are the batteries fully charged at the start and do actually have their intended capacity available? (many chemistries decline with age), how practical would it be to rechange/replace batteries in situ if needed, and the usual handling and transportation precautions.

   - Andy.