There is only one requirement in the standards that calls for maintained emergency lighting and this can be found in BSEN 50172:2004. It outlines the following regulation:
'Maintained emergency light fittings should be used 'If failure of an individual normal luminaire could prejudice safety, maintained emergency luminaires should be used in the area'.'
For non-residential premises used for recreation (such as theatres, cinemas, concert halls, exhibition halls, sports halls, public houses and restaurants) and where the normal lighting might be dimmed or turned off, a maintained or combined emergency escape lighting system should be installed.
However, it is not necessary for the full emergency lighting level to be provided when the normal lighting system is functioning. In case of doubt, the appropriate class as described in BSEN50172:2004 9.2 to 9.11 should be agreed with the enforcing authority.
In all other cases there is no hard and fast legal requirement to provide maintained emergency lighting, however, this can be overruled by the presiding fire officer. Individual risk assessment should always be considered and the fire officer of the region will have the final say.
There is also energy consumption to take into account, if light fittings are left permanently on with no obvious reason then this would constitute a waste of energy.
I would therefore summarise that maintained operation is only required in non-residential premises used for recreation, unless specifically requested by the local fire officer.