It all depends on circumstances and your fire risk assessment.

A shop is OK to have just one door as long as the route through to outside is always clear.

I suggest a fire extinguisher just inside the door as well.

Fire alarm can be a simple shout of "FIRE".

Watch any flammables being stored on or near to the route to the door.

You may wish to keep the rear exit clear as well just in case.

Normally I stay clear of fire-related issues, leaving it to people who can remember travel distances and the different categories of firedoors. But there is one thing that concerns me, “lean-to stores plastic storage boxes but there is a clear path to the final door.” Is there always a clear path to he final door? In my experience it is an unwritten law of storage areas that the amount of stuff in them multiples until it fills the marked out storage areas and spills out on the walkways. If you said the stuff was in cages or behind some sort of barrier I’d be less concerned. How well managed is the storage area? Would you describe the escape route as protected? Remember on this forum we have had arguments over whether putting children’s artwork on a school corridor removes its status as a protected fire escape route.