Windows in non domestic premises have not been accepted as means of escape for many years - in older buildings and with the old Blue & Red books even though they reference them, there were strict constructional aspects & limitaions on who and how many could be expected to use them.
In other words the premises must have sufficient means of escape without recourse to windows. If the premises only have one stair then the liklihood is that they are so small that travel distance & exit width requirements were met with just that stair.
As the sole exit route is protected it's a case of good fire safety management by all that will ensure every one is safe, i.e. free from combustibles & clear of obstructions with closed fire doors.
Fire risk assessment should identify any unusual ignition & combustible hazards that could obstruct the doorway and eliminate them or relocate them.
A type L1 system should provide sufficient early warning as should type H (human!!!) due to the probable small size of the area, allowing for both prompt evacuation, but also prompt application of the means of fighting fire that are/should be provided to the premises for the purposes of securing means of escape.
A trade off has to be reached between security & fire so that one does not severely affect the efficacy of the other, in this case proper fire safety management, construction & systems should mean that the bars can be left if desired.
If the risk is assessed as being so great or on calculation the single exit doen't meet travel etc requirements then you are more likely looking at knocking through a new final exit door, not sending people scrabbling through windows, but I can't see it necessary.
I often get occupiers asking what happens if their single stair/door is compromised, but often point out that it's their failure to comply with the existing precautions that is causing the problem and if they respect the existing structural & alarm provision & actually do a decent FRA following it through with actions & compliance they should be fine
