You are welcome to borrow my lightmeter Davo if you want to conduct a realistic test. But which standard would you apply?
And whilst the BS5266 (Which edition?) sets the benchmark standard to which you should aspire - how is the building used, by whom, at what time and what are the lighting conditions? Is the level of illumination reasonably even along the route? Are points of emphasis covered?
Since you can expect 20% of batteries to fail every year a 5 year plan is eminently reasonable- the trouble is you may end up just replacing old units by similar in the same positions with the same problems ( sub circuit control, lack of test switches, siting to the old 0.2 Lux standard) rather than the newer approach of idenifying and covering the points of emphasis then filling in to an even 1 lux standard.