We have converted two 40 foot shipping containers (joined together along the long side, creating one area approx 40' x 20') in to an Engineering workshop. There is one main point of entry/exit, through a normal style door with a door handle at one end. 

Do we need a 2nd exit door available for fire regs? 

We have left the container doors on one of the containers, which is at the opposite end from the entry door. These cannot be opened from the inside. We have also installed windows in the unit, but these have metal shutters across them externally, although they can by removed, but again only from outside.

I am a bit confused as we have a boardroom which is about the same size with only one exit, for example!

With one door you have an travel in one direction only which means the travel distance should not exceed 18M and the number of occupants must not exceed 60. Also you have created an inner room situation, the factory floor being the access room, so check out https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/14882/fsra-factories-warehouses.pdf  page 73.