I agree with Nearlythere. A fire door is generally an integral part of the means of escape. By wedging it open you probably no longer have sufficient means of escape.
RR(FS)O Article 14(2)(b): in the event of danger, it must be possible for persons to evacuate the premises as quickly and as safely as possible;
Past an open fire door is not really "as safely as possible". It is the same part we would quote as a the non-compliance on an enforcement notice to make someone protect a staircase or escape route with an FR wall/door.
There is also article 8, general fire precautions, linking to article 4...
4. —(1) In this Order "general fire precautions" in relation to premises means, subject to paragraph (2)—
...
(c) measures for securing that, at all material times, the means of escape can be safely and effectively used;