BS 9999 doesn't preclude the use of such a lift and it allows you to 'risk assess' the use of lifts for evacuation provided that you can demonstrate that they offer the same level of safety & functionality as the purpose-designed evacuation lift. Do bear in mind, though, that if the building has a fire alarm and the lift has a modern lift controller, if the fire alarm goes off the lift should automatically go to ground & not respond to landing or car calls (or even key override switches, if fitted) until the fire alarm is successfully reset. Can be difficult to get lift companies to program it any other way, if they don't accept that the lift is a 'compliant' evacuation lift.
To be honest in my experience it's rarely a problem to get the lift to comply with the standard - in an existing building it's associating the lift with an acceptably safe refuge at each level that's the more difficult issue.