Hello, I work in an office with a labelled fire door, leading to the only escape route, but the door is kept locked when we are in the office and the key is kept in a code-locked key-safe to which only 2 people know the code (and no- one who works in the room in question).
I'm hoping you can tell me if this is legal or not - I assume not.
Many thanks.

Is it a "fire door" or is it a "fire exit"

The The Regulatory Reform (Fire Safety) Order 2005 article 14 (f) states " emergency doors must not be so locked or fastened that they cannot be easily and immediately opened by any person who may require to use them in an emergency" I would think your situation does not meet that regulation.

Tom, sorry if I am being pedantic about this, but Mark did say

.........a labelled fire door

He did not say "a labelled fire exit" If it is a fire exit, I would agree with you, but does the door they came in not also count as an exit?

Would it be better to know the size of the office in question and how many people are working in it?

green-foam all doors required for means of escape are emergency doors and doors with signs are only because they are not in normal use and could be used in an emergency. The second door could be required because of the occupancy or travel distance or an alternative MoE but I don't think it is relevant to the question, as all doors that may be required for MoE should be easily and immediately open-able.