When we had the harsh winter last year, all the stocks of grit ran out and then all the shops and supermarkets ran out of salt.
Since then I've added a 1k bag of salt to every online shop. It's the Sainsburys basics stuff and it costs about 17p per bag. Like Leg-Iron says, I'm unsure what they take out of salt to make it cheaper. I've got quite a lot stocked up now for if the same happens again this year, and I've a strong feeling it will.
UK weather forecast for snow and prolonged cold spell
When last nights order arrived at the door, guess what. No salt. They were out of stock.