Vodafone 100GB Data + unlimited mins & txts - £8.50 per month (12 Month SIM)
This is the best deal I have found for SIM only on Vodafone as I am looking for myself. It's a lot better than the one being advertised by Latest Deals staff because this SIM has global roaming included. It is not one of those Vodafone Basics contracts.
The contract costs £16 per month.
You get £50 cashback from Carphone Warehouse (automatic)
You get £40 cashback from Topcashback (automatic)
£16 x 12 = £192
£192 - £50 - £40 = £102
£102 / 12 = £8.50
Make sure you click through from the Topcashback page I have linked to or it will not work. You need a Topcashback account too.