UK laws regarding consumption of food or beverages in a commercial kitchen?
About the only perk to being a chef is eating the fancy foods, which you have not even the dream of being able to afford on the measly less than minimum wage you get paid for slaving over rich ungrateful customers food night in night out. 

No safety law I've ever heard of, probably just some over cautious owner trying to increase their bottom line by ensuring nothing is ever eaten or drank that may possibly cost them any pennies. 

In one place I worked, we had to learn to shove full sandwiches in our mouths, so the boss didn't catch us eating during our 14 hour shifts (with no staff food or dinner breaks).