The original price of this deal is £20 per month, but £138 cashback drops the price to under a tenner a month. You will have to claim the cashback at specific intervals through the 12-month term upon receiving your monthly bill, which you can do via a Mobiles account.
While Affordable Mobile's £8/60GB SIM isn't available, it is worth taking a look at its Vodafone Red Entertainment plan, also with 60GB of data at 5G speeds on a 12-month term. It costs £5.50 more, which translates to £13.50 per month, but you also get a free 1-year subscription to either Spotify Premium, Amazon Prime Video, Now TV or Sky Sports. See this Entertainment plan over at Affordable Mobiles.