I recently got a sim only deal with O2, £22 a month for 25gb of data and unlimited calls and messages, I have poor credit too and still got accepted, signal can be poor in places at times but it builds ur credit as it’s a contract. Giffgaff uses the O2 network and is quite good although you don’t get the support you would by going direct to O2.
I used to be on giffgaff though and can’t knock it, it’s simple to top up and you can just set your goodybag to a recurring payment so you don’t have to worry about manually topping up
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