No a legal requirement for the number, but there are guidelines issued by Firesafe - the number they recommend depends on the type of environment you work in. An office would clearly warrant fewer drills than, say, a manufacturing facility where flammable gasses etc. are in use.
If you refused to comply with the drill I'd expect you'd be in breach of contract - the part about complying with any reasonable requests made of you (and no matter what you think, a fire drill is a reasonable request). Whether it was misconduct or gross-misconduct would probably depend on the terms and policies of your employer, but I'd expect gross-misconduct. If you were preventing your employer from carrying out adequate fire safety training, thereby getting them in trouble potentially with the fire brigade, Health & Safety Executive and their insurance companies, they'd definitely have reasonable grounds to sack you.
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