callum9999:
They'll be quaking in their boots!
On the plus side, customers threatening to report you to trading standards for ridiculous things like this (you make it sound like there's a law saying they must have such a notice - I've never seen it?) invariably brightened my day back when I had to work in a supermarket, so they may get an amusing anecdote to share with their colleagues!
I wouldn't make any threats in store. I've worked in retail and didn't appreciate that behaviour when I was on the serving side of the till so I don't do it when I'm on the paying side of the till.
With the legislation I quoted, I'm not convinced it's a definitive solution but if you're going to put a restriction on the number of items that can be bought, you are supposed to put signage up making that clear to customers at the point of sale. Under the legislation I cited, there is an argument (albeit a stretched argument) for saying it could be a misleading omission and as such potentially a matter for Trading Standards. As always YMMV.
1F