Article 15 of the Fire Safety Order states:
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15.—(1) The responsible person must—
(a)establish and, where necessary, give effect to appropriate procedures, including safety drills, to be followed in the event of serious and imminent danger to relevant persons;
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Note: I have highlighted 'where necessary' in bold. This obviously means there are some circumstances where fire drills are not required/necessary.
Imagine a husband an wife retail local grocery store with an alcohol licence. They have a FRA, they may well have agreed a plan of who does what, but would you really expect them to carry out a drill?
Likewise, when you do conduct drills, there is no frequency dictated in the Fire Safety Order. It is best practice - especially as its mentioned in the Govt FS Guides - to conduct drills annually. Many Resp Persons assess that more frequent drills are required, say in residential homes and schools where specific high risk or vulnerable persons are present.
IMHO, it would be difficult to justify a period of more than 12 months as a part of your fire safety strategy, but if for some reason the gap is extended by 2 months due to unforeseen operational or other circumstances, I really cannot see an enforcement authority making too much of a fuss.
During covid, we missed our head office fire drill for nearly two years. Instead we provided online training and published extra guidance on our intranet. Last year, we had an office close to where 1000s of people were queuing to see HM The Queen laying in state. We delayed the fire drill as it seemed a bit crass turning out many 100s of staff into the queue area.
I would apply the same common sense if your firm had a large order and that delayed the drill by a few weeks
So to answer your question, there is no legal requirement to conduct annual drills. However, as with the best practice published in the Govt FS Guides, British standards and Approved doc B of Building Regs, you may have to justify why it was necessary to not apply those standards - especially post incident