There exist quite a few different SIM Only plans, and each one is designed to appeal to the needs of very different mobile customers. For example, you can see that 3, Orange, Virgin Mobile, O2, T-Mobile and Vodafone all offer their own choices, which works well for customers because it means low prices, healthy competition and great allowances.
These plans are built directly into the SIM card that is already present on your phone, and the allowances you receive will depend on your chosen deal....