We have just had some major renovations done at one of our offices including new rooms being built etc. One of these is a store room, inside is a mains distribution board, but i am shocked that the door is NOT a fire door. My question is - should it be?

The room stores cardboard stock and spare furniture

Building Regulations are a good starting point:

Refer to Table C1 on page 168 of Approved Document B

https://assets.publishing.service.gov.uk/media/67d17064a005e6f9841a1d50/Approved_Document_B_volume_2_Buildings_other_than_Dwellings_2019_edition_incorporating_2020_2022_and_2025_amendments_collated_with_2026_and_2029_amendments.pdf

A storeroom does not necessarily need a fire door, it all depends on the flammability of what is so be stored in the room, cardboard and spare furniture I would not regard as a major risk as long as it is controlled and does not become a dumping ground.

If the distribution board is in a fire rated enclosure, then there is no need to put it behind a further fire door.

Use a camera to take a picture of the top of the door, it may be a fire door, just not set up as one as the cost of the doors are not that different.  The major difference is the sealing and inspection.