EL in existing premises where none exists

[LWSElectrical]

Hi, I'm after some advice regarding EL in existing premises where there is no EL at present. It's a medium size warehouse (38m X 20m) with polycarbonate roof sheets so plenty of natural daylight apart from late afternoon Nov-Jan. The shifts finish at 17:00-17:30 so EL is only required for 3 months of the year for a couple of hours at most. 
They have final exits, signage, fire extinguishers etc but no EL at present. The customer says that according to their fire safety "expert" they only need a couple of ELs above the fire exits as the signage is photo-luminescent. Does this sound right or should New EL installations in older buildings conform fully to BS5266-1:2016. They don't want to pay out for a fully compliant install. I'm wondering is it better to do nothing than to not comply and face serious consequences should the worst happen when the EL is needed? Is it acceptable to deviate from BS5266-1:2016 if a risk assessment has been signed and agreed? Thanks in advance for your help

[AnthonyB]

Not much of an expert......  Photoluminescent signage cannot compensate for lighting unless it's a full BS5266-6:1999 compliant low mounted way guidance system such as the below:
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which ensures the safe route is illuminated. You need a good continuous supply of light (usually white light fluorescent or LED as oppose to natural light) to 'charge' the photoluminescent material so it will glow at the required lighting level for the required duration on light failiure

EL provision is risk assessment based as acknowledged in the current BS5266-1 and so an FRA could require more cover or even less.

It's OK to use borrowed or natural light but you must bear in mind that natural light is strongly susceptible to the time of year, time of day and even the weather and you need to account for all times relevant persons may be expected to be on site and the possible risks. Don't forget that EL isn't just a fire safety thing, but a general H&S one - it's a far more common occurrence for circuit and power failure when there isn't a fire - hence why many older existing premises are gradually upping their EL cover