It may sound obvious but couldn't the person who locks up just pop his head around the door and check no one is in before locking up, or keep a spare key on the inside wall (near the door).

These are temporary buildings, intended for short term use. Therefore I would RA it as acceptable due to the short term temporary installation. There is no requirement for a panic bar opening handle as it's low occupation, only used by those familiar with the layout and wouldn't be involved in a 'mas evacuation' scenario. If you're concerned about the door being locked from outside, (and likely is that?) then place a spare key inside in a glass key holder? It's about significant risks and likelihood isn't it?

And no fire exits don't HAVE to open outwards. Only if it was a mass evacuation where people would be trapped behind a door would it be necessary. Sometimes opening outwards can be a hazard itself if it opens onto another exit route.

Just quick note on fire exits opening outwards; they DO have to open in direction of travel are there is a capacity limit on use of such, but with a portakabin this capacity would not be exceeded.

Fire Exits do not HAVE to open outwards. That used to be the case many years ago. But since the demise of the FP Act and Firs Servies Act it's all down to RA by the occupier. In cases where the hazard of a door opening outwards would block another escape route it would be more than acceptable to have it open inwards. Also it would depend on the number of people possibly using it in a hurry. I would suggest that a standard size portacabin is not going to hold a great deal of people and those people are aware of the escape route and method of operating the door, therefore the door opening inwards is not a problem. Scenario:- You rearrange the offices in your building. Old one had an outward opening door as the building was built 20 years ago. New office, in different part of building has door that opens inwards as it was never an occupied area and not an escape route. However the office only has 6 people in it. Where would be the hazard in it opening inwards? It would be unreasonable on grounds of cost to have the door taken out, rebuilt and reversed for such a low risk. If however you have lots of people in the office, there is a chance of mass evacuation being required and some occupants would be unfamiliar with the escape, (public, visitors etc.) then YES have it open outwards, because there is now a chance of people becoming trapped behind the door. I'll stop now because I'm losing the thread....

Why not have a standard lever type door handle. Much easier to open for everyone and even more 'simples' than a thumb turn?