For better or for worse, the signage is mandatory, and Mr. Beasley knew or ought properly to have known that this was mandatory.
We would not countenance him driving without other mandatory equipment, so what is it about the sign that is in a different category of mandatory from, say, safety equipment, number plates, insurance coverage, or any of the other things that the law requires be in place before putting a commercial vehicle on the road?
As for the magnitude of the fine, I suspect that we are in the realm of legislation that creates a basic level of fine for any breach that is not specifically assigned a higher penalty. And given that this legislation is directed at business, the level of fine appears reasonable for a regulatory breach.
On the other hand, the smoking ban itself is directed at individual conduct, in which £200 would be considered far less reasonable. Comparing the penalty applicable to a business for failure to comply with regulations, and a penalty applicable to an individual for a contravention might serve to explain the difference in magnitude.
And as for nanny-statism. Well, workplace safety is a tough nut to crack. Are mandatory hard hats on contruction sites nanny-statism? Where is the line to be drawn? The signage requirement might go too far, but on the other hand, when business complies with mandatory signage, then no one can pretend, "I didn't know that I couldn't light up."
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--James