Upon covering all the areas from the short guide, you must design any additional lights to ensure that minimum lux levels are met to enable safe evacuation. Moreover, every section on the escape route must have at least two luminaires, to continue to offer light in the event of one of the failing.
BS 5266 Pt 7: 1999 (EN1838) specifies a minimum of 1 lux anywhere on the centre line of the escape route for normal risks. This level must be continued for the full duration of the failure.
This is for guidence only and whatever your specific situation it is always best to seek advice from a qualified electrical installer and your local fire officer to ensure that you comply with the local and national laws.