A couple cashback options available here from mobile provider GiffGaff. The simplest one is to just order a free Sim card, and you’ll get 50p cashback for free. This is available from Quidco and Topcashback.
Alternatively if you register and top up your newly acquired Sim card with £10, you will receive £14 cashback from Topcashback. That £4 profit, and obviously the £10 of mobile usage you’ve just purchased. There’s no contract so can simply throw away the Sim after you’ve run out of credit if you no longer wish to continue with GiffGaff. however, the company does also get excellent reviews so you may just stick with it!