Oh dear, seems that everyone may have missed the most important point here. A tin or whatever removed from a shrink wrapped multipack has one key thing missing: a barcode.
No matter how they try or how unhelpful a supermarket cashier is being, they cannot scan a product with no barcode. It's a simple as that. Whilst individual packs in a multipack do indeed omit key information such as ingredients, allergens and weight, they have nothing to do with the saleability of the product and these omissions are red-herrings.
The price a supermarket values a product can vary terrifically depending on whether it was bought in as part of a multipack or as a single container. Sometimes the wholesale purchase price can vary between the two by 1000% or more (yes, I did say thousand percent) when promotions and other methods are agreed between the supermarket buyer and the manufacturer.
Because of this, supermarkets are in no rush to sell you an unbarcoded product despite the fact that the profit on that item can be massive compared to an identical barcoded item.
Finally, no it's not illegal to sell multipack items singly regardless if the store use scanners only or rings up the item on the till. It's just easier not to as it can create problems in stocktaking etc.
Incidentally, most goods-inwards managers at supermarkets are forbidden from accepting deliveries if the product fails to have a barcode.