Tricky set of questions Harris, I'll put down my twopennorth but I'm sure there will be more to be said:

Fire doors are used to act as boundaries of compartments and also to protect escape routes. In some environments particular rooms will automatically have fire doors; for example in healthcare premises a boiler room would have a fire door as it would be classed as a fire hazard room, in other premises that would be down to fire risk assessment. Their primary purpose is to protect the occupants of the building from the effects of smoke and flames for long enough for them to escape. They may also act to restrict a fire to one part of a building if the fire service get there in time to put out the fire before it breaches the compartment.

There aren't really any regs on compartmentalisation; building regulations will tell you how to build a compartment, and give an idea of compartment size in relation to travel distances and occupancy, but there aren't any regs that say for x size of building you need y compartments. A major consideration in a large building is distance to a place of relative safety; this would be the next fire compartment to the one that's on fire. Fire suppression systems (sprinklers) are used in buildings where travel distances will be large, such as warehouses. It is all down to FRA.

Would a FRA consider compartmentalisation? This is a discussion we're having with our property services at the moment. We say, yes it should, they say, that's not what the contract with the FRA provider asks for. Me, I acn't imagine why you would do an FRA and not consider your compartments,

John

In addition to the comments made, fire doors/compartmentation may also be installed for property protection.

The evacuation strategy may dictate as well e.g. blocks of purpose built flats.

There are many guides which are sector specific which will include provision of fire doors e.g. BB101 for schools, HTM "Firecode" for healthcare 

https://www.gov.uk/government/publications/fire-safety-approved-document-b

Updated Approved Document B for the Building Regulations

To confirm compartmentalisation etc sometimes this is caveated by the FRA stating it was based on non intrusive inspection- they may pick up on obvoius breaches but won't take off panelling, go into ceiling voids etc- there are different levels of FRA, and i know you can ask get just compartmetalisation assessments which use boroscopes to inspect.

https://www.gov.uk/government/publications/fire-safety-approved-document-b?utm_source=974c8414-e065-46a9-ab55-b2fe12eeda7a&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate

5th July sees the 2019 version of Approved Document B published which takes effect August