I was not aware that was the law in the U.K. I would probably have found out by being nice. Less likely to find out by being abrasive.
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The probably-underpaid people who break policy make the world go around. We'd get nothing real done if people just followed policy all day long.
It might sound like I'm joking, but I don't. I have huge amounts of respect for those people, and obviously if they help me out I will do what I can to help them out. (Even if it's just paying it forward and breaking policy when necessary myself.)
I'm not anti-policy. I love good policy, and I rarely have reason to break it. The policy that often needs breaking is the bad policy.
In general, I think it's unfair to ask a probably-underpaid cashier to break policy in this way -- they almost certainly have no control over the policy, and quite possibly face severe consequences if they acquiesce.