Three have various sim only deals and you get £50 amazon voucher plus possible cashback from quidco.
One example of sim deals is 200 minutes, unlimited text's, unlimited data (subject to £30GB personal hotspot cap). £23 on 12 month contract. Included is £50 Amazon voucher and possible cashback from Quidco.
They have others with more minutes, less data etc to suit your needs so play around with the Sims section.
There is all kinds of other deals that include the gift card, at the time of writing, it's on 13 days before the deal expires.