This saves me an absurd amount of planning/thinking as I can compare an equivalent cost number whenever I see a substitutable product somewhere. Also saves you from being fooled by your own assumptions, like when the unit cost of the largest cereal box is higher than a smaller one.

I thought this was already mandated by US law, but ...

In some cases they are selling (for £1) a product that your supermarket pays 70p for and sometimes sells for £1.50 and other times "discounts" to 75p. If you bought it from the Pound Shop when it's £1.50 at a supermarket that seems like a bargain, if it was 75p at the supermarket it seems like a rip off. But it's the same product either way.

In some cases as you described they have a 300g version of a 450g product, theirs costs £1 and the "ordinary" version is £1.40 from many retailers. So they are more expensive on a "per-unit" basis. But let's face it, you were going to eat the entire product even though the manufacturer says it's for "sharing", so maybe they did you a favour.

In some cases the manufacturer is engaged in simple price discrimination. The pound store has the exact same quantity of the branded product, but it's wrapped with a label explaining that it only cost £1 whereas the version in the supermarket is unlabelled and costs £1.50. The manufacturer guesses that many people who can afford £1.50 will choose not to visit a Pound Shop, so any sales at the lower price are extra sales and don't cannibalise supermarket sales.

In some cases they're End of Line goods. The supermarket doesn't want to stock a product with a finite supply, especially out of season - it looks desperate, but a Pound Shop can put boxes of "Santa Shapes" candy on shelves in January for £1 (original RRP £2.50) and customers will buy them because candy is candy, who cares if it's Xmas themed when it's so cheap? Or after supermarkets pull the £1.85 "barbecue flavour" chocolate assortment you launched because it doesn't sell, bundle them two for £1 at a Pound Shop and they disappear. Do the Pound Shop's customers eat them? Who cares.

The final category that's most interesting to me is bulk electronics. If you stock sixteen different adaptors in an electronics shop, they move so slowly you probably have to charge £2.50 or more for them even though they're quite cheap to make. But a Pound Shop can pick one adaptor they think sells (say, USB A to USB C is plausible) buy an entire container of them, put them on shelves for a few weeks at £1 each, and they vanish. Want that USB A to C adaptor a month later? Too bad, now they have a GU10 LED lamp, and a month after that maybe it's a laser pointer. Could I get these products on Amazon any time? Yeah, but either the postage kills the deal, or it's "free" and the price is higher.