They don't offer to do the contracts anymore in an Apple store. Imagining keeping SIMs for 5 networks and having information of every tariff etc with the added paperwork? They offer to help set you up. I saw the poster offering to help set people up with their carriers, although with most people having the much bigger original SIMs - I don't know what this help actually entails.
However to the point of Tescos - you need to go into a Tesco Extra store and pick it up personally. You cannot get a Tesco contract (or any other) at any Apple store.