As I say, I'm not sure where you shop but where I shop you can.

A Single loose King Edward is 38p

A single red onion is 23p

The other things you mentioned do come in singles for a few varieties but there isn't a big enough demand for single pointed peppers to justify a large area on a shelf so those come in packs of two. I don't think it's unreasonable for a large supermarket not to sell loose chillies if they offer then in 50g bags.

If it's that much of a problem you could always vote with your wallet and go to a local farmers market? With the exception of some tiny bits of plastic the vast majority of Waitrose's packaging is recyclable.